Combine OS-independent ABD Code into Common Source File

Reorganizing ABD code base so OS-independent ABD code has been placed
into a common abd.c file. OS-dependent ABD code has been left in each
OS's ABD source files, and these source files have been renamed to
abd_os.

The OS-independent ABD code is now under:
module/zfs/abd.c
With the OS-dependent code in:
module/os/linux/zfs/abd_os.c
module/os/freebsd/zfs/abd_os.c

Reviewed-by: Matthew Ahrens <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Brian Atkinson <[email protected]>
Closes #10293 

12 files changed, 1602 insertions, 2071 deletions

diff --git a/include/sys/Makefile.am b/include/sys/Makefile.am
index 82165170a..17d78658c 100644
--- a/include/sys/Makefile.am
+++ b/include/sys/Makefile.am
@@ -2,6 +2,7 @@ SUBDIRS = fm fs crypto lua sysevent
 
 COMMON_H = \
 	$(top_srcdir)/include/sys/abd.h \
+	$(top_srcdir)/include/sys/abd_impl.h \
 	$(top_srcdir)/include/sys/aggsum.h \
 	$(top_srcdir)/include/sys/arc.h \
 	$(top_srcdir)/include/sys/arc_impl.h \
diff --git a/include/sys/abd.h b/include/sys/abd.h
index 82b73589b..df4234f3c 100644
--- a/include/sys/abd.h
+++ b/include/sys/abd.h
@@ -35,56 +35,14 @@
 extern "C" {
 #endif
 
-typedef enum abd_flags {
-	ABD_FLAG_LINEAR	= 1 << 0,	/* is buffer linear (or scattered)? */
-	ABD_FLAG_OWNER	= 1 << 1,	/* does it own its data buffers? */
-	ABD_FLAG_META	= 1 << 2,	/* does this represent FS metadata? */
-	ABD_FLAG_MULTI_ZONE  = 1 << 3,	/* pages split over memory zones */
-	ABD_FLAG_MULTI_CHUNK = 1 << 4,	/* pages split over multiple chunks */
-	ABD_FLAG_LINEAR_PAGE = 1 << 5,	/* linear but allocd from page */
-} abd_flags_t;
-
-typedef struct abd {
-	abd_flags_t	abd_flags;
-	uint_t		abd_size;	/* excludes scattered abd_offset */
-	struct abd	*abd_parent;
-	zfs_refcount_t	abd_children;
-	union {
-		struct abd_scatter {
-			uint_t		abd_offset;
-#if defined(__FreeBSD__) && defined(_KERNEL)
-			uint_t  abd_chunk_size;
-			void    *abd_chunks[];
-#else
-			uint_t		abd_nents;
-			struct scatterlist *abd_sgl;
-#endif
-		} abd_scatter;
-		struct abd_linear {
-			void		*abd_buf;
-			struct scatterlist *abd_sgl; /* for LINEAR_PAGE */
-		} abd_linear;
-	} abd_u;
-} abd_t;
+struct abd; /* forward declaration */
+typedef struct abd abd_t;
 
 typedef int abd_iter_func_t(void *buf, size_t len, void *private);
 typedef int abd_iter_func2_t(void *bufa, void *bufb, size_t len, void *private);
 
 extern int zfs_abd_scatter_enabled;
 
-static inline boolean_t
-abd_is_linear(abd_t *abd)
-{
-	return ((abd->abd_flags & ABD_FLAG_LINEAR) != 0 ? B_TRUE : B_FALSE);
-}
-
-static inline boolean_t
-abd_is_linear_page(abd_t *abd)
-{
-	return ((abd->abd_flags & ABD_FLAG_LINEAR_PAGE) != 0 ?
-	    B_TRUE : B_FALSE);
-}
-
 /*
  * Allocations and deallocations
  */
@@ -124,12 +82,8 @@ void abd_copy_to_buf_off(void *, abd_t *, size_t, size_t);
 int abd_cmp(abd_t *, abd_t *);
 int abd_cmp_buf_off(abd_t *, const void *, size_t, size_t);
 void abd_zero_off(abd_t *, size_t, size_t);
-
-#if defined(_KERNEL)
-unsigned int abd_scatter_bio_map_off(struct bio *, abd_t *, unsigned int,
-		size_t);
-unsigned long abd_nr_pages_off(abd_t *, unsigned int, size_t);
-#endif
+void abd_verify(abd_t *);
+uint_t abd_get_size(abd_t *);
 
 void abd_raidz_gen_iterate(abd_t **cabds, abd_t *dabd,
 	ssize_t csize, ssize_t dsize, const unsigned parity,
@@ -175,12 +129,28 @@ abd_zero(abd_t *abd, size_t size)
 }
 
 /*
+ * ABD type check functions
+ */
+boolean_t abd_is_linear(abd_t *);
+boolean_t abd_is_linear_page(abd_t *);
+
+/*
  * Module lifecycle
+ * Defined in each specific OS's abd_os.c
  */
 
 void abd_init(void);
 void abd_fini(void);
 
+/*
+ * Linux ABD bio functions
+ */
+#if defined(__linux__) && defined(_KERNEL)
+unsigned int abd_scatter_bio_map_off(struct bio *, abd_t *, unsigned int,
+    size_t);
+unsigned long abd_nr_pages_off(abd_t *, unsigned int, size_t);
+#endif
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/include/sys/abd_impl.h b/include/sys/abd_impl.h
new file mode 100644
index 000000000..6027678af
--- /dev/null
+++ b/include/sys/abd_impl.h
@@ -0,0 +1,126 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://www.opensolaris.org/os/licensing.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+/*
+ * Copyright (c) 2014 by Chunwei Chen. All rights reserved.
+ * Copyright (c) 2016, 2019 by Delphix. All rights reserved.
+ */
+
+#ifndef _ABD_IMPL_H
+#define	_ABD_IMPL_H
+
+#include <sys/abd.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef enum abd_flags {
+	ABD_FLAG_LINEAR		= 1 << 0, /* is buffer linear (or scattered)? */
+	ABD_FLAG_OWNER		= 1 << 1, /* does it own its data buffers? */
+	ABD_FLAG_META		= 1 << 2, /* does this represent FS metadata? */
+	ABD_FLAG_MULTI_ZONE  	= 1 << 3, /* pages split over memory zones */
+	ABD_FLAG_MULTI_CHUNK 	= 1 << 4, /* pages split over multiple chunks */
+	ABD_FLAG_LINEAR_PAGE 	= 1 << 5, /* linear but allocd from page */
+} abd_flags_t;
+
+typedef enum abd_stats_op {
+	ABDSTAT_INCR, /* Increase abdstat values */
+	ABDSTAT_DECR  /* Decrease abdstat values */
+} abd_stats_op_t;
+
+struct abd {
+	abd_flags_t	abd_flags;
+	uint_t		abd_size;	/* excludes scattered abd_offset */
+	struct abd	*abd_parent;
+	zfs_refcount_t	abd_children;
+	union {
+		struct abd_scatter {
+			uint_t		abd_offset;
+#if defined(__FreeBSD__) && defined(_KERNEL)
+			uint_t  abd_chunk_size;
+			void    *abd_chunks[];
+#else
+			uint_t		abd_nents;
+			struct scatterlist *abd_sgl;
+#endif
+		} abd_scatter;
+		struct abd_linear {
+			void		*abd_buf;
+			struct scatterlist *abd_sgl; /* for LINEAR_PAGE */
+		} abd_linear;
+	} abd_u;
+};
+
+struct scatterlist; /* forward declaration */
+
+struct abd_iter {
+	/* public interface */
+	void		*iter_mapaddr;	/* addr corresponding to iter_pos */
+	size_t		iter_mapsize;	/* length of data valid at mapaddr */
+
+	/* private */
+	abd_t		*iter_abd;	/* ABD being iterated through */
+	size_t		iter_pos;
+	size_t		iter_offset;	/* offset in current sg/abd_buf, */
+					/* abd_offset included */
+	struct scatterlist *iter_sg;	/* current sg */
+};
+
+/*
+ * OS specific functions
+ */
+
+abd_t *abd_alloc_struct(size_t);
+abd_t *abd_get_offset_scatter(abd_t *, size_t);
+void abd_free_struct(abd_t *);
+void abd_alloc_chunks(abd_t *, size_t);
+void abd_free_chunks(abd_t *);
+boolean_t abd_size_alloc_linear(size_t);
+void abd_update_scatter_stats(abd_t *, abd_stats_op_t);
+void abd_update_linear_stats(abd_t *, abd_stats_op_t);
+void abd_verify_scatter(abd_t *);
+void abd_free_linear_page(abd_t *);
+void abd_enter_critical(unsigned long);
+void abd_exit_critical(unsigned long);
+/* OS specific abd_iter functions */
+void abd_iter_init(struct abd_iter  *, abd_t *);
+boolean_t abd_iter_at_end(struct abd_iter *);
+void abd_iter_advance(struct abd_iter *, size_t);
+void abd_iter_map(struct abd_iter *);
+void abd_iter_unmap(struct abd_iter *);
+
+/*
+ * Helper macros
+ */
+#define	ABDSTAT(stat)		(abd_stats.stat.value.ui64)
+#define	ABDSTAT_INCR(stat, val) \
+	atomic_add_64(&abd_stats.stat.value.ui64, (val))
+#define	ABDSTAT_BUMP(stat)	ABDSTAT_INCR(stat, 1)
+#define	ABDSTAT_BUMPDOWN(stat)	ABDSTAT_INCR(stat, -1)
+
+#define	ABD_SCATTER(abd)	(abd->abd_u.abd_scatter)
+#define	ABD_LINEAR_BUF(abd)	(abd->abd_u.abd_linear.abd_buf)
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif	/* _ABD_IMPL_H */
diff --git a/lib/libzpool/Makefile.am b/lib/libzpool/Makefile.am
index a9396105b..0e6a1058e 100644
--- a/lib/libzpool/Makefile.am
+++ b/lib/libzpool/Makefile.am
@@ -39,6 +39,7 @@ KERNEL_C = \
 	zpool_prop.c \
 	zprop_common.c \
 	abd.c \
+	abd_os.c \
 	aggsum.c \
 	arc.c \
 	arc_os.c \
diff --git a/module/Makefile.bsd b/module/Makefile.bsd
index 6d76796f5..92b5c1906 100644
--- a/module/Makefile.bsd
+++ b/module/Makefile.bsd
@@ -127,7 +127,7 @@ SRCS+= spl_atomic.c
 .endif
 
 #os/freebsd/zfs
-SRCS+=	abd.c \
+SRCS+=	abd_os.c \
 	crypto_os.c \
 	dmu_os.c \
 	hkdf.c \
@@ -169,7 +169,8 @@ SRCS+=	zfeature_common.c \
 	zprop_common.c
 
 #zfs
-SRCS+=	aggsum.c \
+SRCS+=	abd.c \
+	aggsum.c \
 	arc.c \
 	arc_os.c \
 	blkptr.c \
diff --git a/module/os/freebsd/zfs/abd_os.c b/module/os/freebsd/zfs/abd_os.c
new file mode 100644
index 000000000..f438841cd
--- /dev/null
+++ b/module/os/freebsd/zfs/abd_os.c
@@ -0,0 +1,433 @@
+/*
+ * This file and its contents are supplied under the terms of the
+ * Common Development and Distribution License ("CDDL"), version 1.0.
+ * You may only use this file in accordance with the terms of version
+ * 1.0 of the CDDL.
+ *
+ * A full copy of the text of the CDDL should have accompanied this
+ * source.  A copy of the CDDL is also available via the Internet at
+ * http://www.illumos.org/license/CDDL.
+ */
+
+/*
+ * Copyright (c) 2014 by Chunwei Chen. All rights reserved.
+ * Copyright (c) 2016 by Delphix. All rights reserved.
+ */
+
+/*
+ * See abd.c for a general overview of the arc buffered data (ABD).
+ *
+ * Using a large proportion of scattered ABDs decreases ARC fragmentation since
+ * when we are at the limit of allocatable space, using equal-size chunks will
+ * allow us to quickly reclaim enough space for a new large allocation (assuming
+ * it is also scattered).
+ *
+ * ABDs are allocated scattered by default unless the caller uses
+ * abd_alloc_linear() or zfs_abd_scatter_enabled is disabled.
+ */
+
+#include <sys/abd_impl.h>
+#include <sys/param.h>
+#include <sys/zio.h>
+#include <sys/zfs_context.h>
+#include <sys/zfs_znode.h>
+
+typedef struct abd_stats {
+	kstat_named_t abdstat_struct_size;
+	kstat_named_t abdstat_scatter_cnt;
+	kstat_named_t abdstat_scatter_data_size;
+	kstat_named_t abdstat_scatter_chunk_waste;
+	kstat_named_t abdstat_linear_cnt;
+	kstat_named_t abdstat_linear_data_size;
+} abd_stats_t;
+
+static abd_stats_t abd_stats = {
+	/* Amount of memory occupied by all of the abd_t struct allocations */
+	{ "struct_size",			KSTAT_DATA_UINT64 },
+	/*
+	 * The number of scatter ABDs which are currently allocated, excluding
+	 * ABDs which don't own their data (for instance the ones which were
+	 * allocated through abd_get_offset()).
+	 */
+	{ "scatter_cnt",			KSTAT_DATA_UINT64 },
+	/* Amount of data stored in all scatter ABDs tracked by scatter_cnt */
+	{ "scatter_data_size",			KSTAT_DATA_UINT64 },
+	/*
+	 * The amount of space wasted at the end of the last chunk across all
+	 * scatter ABDs tracked by scatter_cnt.
+	 */
+	{ "scatter_chunk_waste",		KSTAT_DATA_UINT64 },
+	/*
+	 * The number of linear ABDs which are currently allocated, excluding
+	 * ABDs which don't own their data (for instance the ones which were
+	 * allocated through abd_get_offset() and abd_get_from_buf()). If an
+	 * ABD takes ownership of its buf then it will become tracked.
+	 */
+	{ "linear_cnt",				KSTAT_DATA_UINT64 },
+	/* Amount of data stored in all linear ABDs tracked by linear_cnt */
+	{ "linear_data_size",			KSTAT_DATA_UINT64 },
+};
+
+/*
+ * The size of the chunks ABD allocates. Because the sizes allocated from the
+ * kmem_cache can't change, this tunable can only be modified at boot. Changing
+ * it at runtime would cause ABD iteration to work incorrectly for ABDs which
+ * were allocated with the old size, so a safeguard has been put in place which
+ * will cause the machine to panic if you change it and try to access the data
+ * within a scattered ABD.
+ */
+size_t zfs_abd_chunk_size = 4096;
+
+#if defined(_KERNEL)
+SYSCTL_DECL(_vfs_zfs);
+
+SYSCTL_INT(_vfs_zfs, OID_AUTO, abd_scatter_enabled, CTLFLAG_RWTUN,
+	&zfs_abd_scatter_enabled, 0, "Enable scattered ARC data buffers");
+SYSCTL_ULONG(_vfs_zfs, OID_AUTO, abd_chunk_size, CTLFLAG_RDTUN,
+	&zfs_abd_chunk_size, 0, "The size of the chunks ABD allocates");
+#endif
+
+kmem_cache_t *abd_chunk_cache;
+static kstat_t *abd_ksp;
+
+static void
+abd_free_chunk(void *c)
+{
+	kmem_cache_free(abd_chunk_cache, c);
+}
+
+static size_t
+abd_chunkcnt_for_bytes(size_t size)
+{
+	return (P2ROUNDUP(size, zfs_abd_chunk_size) / zfs_abd_chunk_size);
+}
+
+static inline size_t
+abd_scatter_chunkcnt(abd_t *abd)
+{
+	ASSERT(!abd_is_linear(abd));
+	return (abd_chunkcnt_for_bytes(
+	    ABD_SCATTER(abd).abd_offset + abd->abd_size));
+}
+
+boolean_t
+abd_size_alloc_linear(size_t size)
+{
+	return (size <= zfs_abd_chunk_size ? B_TRUE : B_FALSE);
+}
+
+void
+abd_update_scatter_stats(abd_t *abd, abd_stats_op_t op)
+{
+	size_t n = abd_scatter_chunkcnt(abd);
+	ASSERT(op == ABDSTAT_INCR || op == ABDSTAT_DECR);
+	if (op == ABDSTAT_INCR) {
+		ABDSTAT_BUMP(abdstat_scatter_cnt);
+		ABDSTAT_INCR(abdstat_scatter_data_size, abd->abd_size);
+		ABDSTAT_INCR(abdstat_scatter_chunk_waste,
+		    n * zfs_abd_chunk_size - abd->abd_size);
+	} else {
+		ABDSTAT_BUMPDOWN(abdstat_scatter_cnt);
+		ABDSTAT_INCR(abdstat_scatter_data_size, -(int)abd->abd_size);
+		ABDSTAT_INCR(abdstat_scatter_chunk_waste,
+		    abd->abd_size - n * zfs_abd_chunk_size);
+	}
+}
+
+void
+abd_update_linear_stats(abd_t *abd, abd_stats_op_t op)
+{
+	ASSERT(op == ABDSTAT_INCR || op == ABDSTAT_DECR);
+	if (op == ABDSTAT_INCR) {
+		ABDSTAT_BUMP(abdstat_linear_cnt);
+		ABDSTAT_INCR(abdstat_linear_data_size, abd->abd_size);
+	} else {
+		ABDSTAT_BUMPDOWN(abdstat_linear_cnt);
+		ABDSTAT_INCR(abdstat_linear_data_size, -(int)abd->abd_size);
+	}
+}
+
+void
+abd_verify_scatter(abd_t *abd)
+{
+	/*
+	 * There is no scatter linear pages in FreeBSD so there is an
+	 * if an error if the ABD has been marked as a linear page.
+	 */
+	VERIFY(!abd_is_linear_page(abd));
+	ASSERT3U(ABD_SCATTER(abd).abd_offset, <,
+	    zfs_abd_chunk_size);
+	size_t n = abd_scatter_chunkcnt(abd);
+	for (int i = 0; i < n; i++) {
+		ASSERT3P(
+		    ABD_SCATTER(abd).abd_chunks[i], !=, NULL);
+	}
+}
+
+void
+abd_alloc_chunks(abd_t *abd, size_t size)
+{
+	size_t n = abd_chunkcnt_for_bytes(size);
+	for (int i = 0; i < n; i++) {
+		void *c = kmem_cache_alloc(abd_chunk_cache, KM_PUSHPAGE);
+		ASSERT3P(c, !=, NULL);
+		ABD_SCATTER(abd).abd_chunks[i] = c;
+	}
+	ABD_SCATTER(abd).abd_chunk_size = zfs_abd_chunk_size;
+}
+
+void
+abd_free_chunks(abd_t *abd)
+{
+	size_t n = abd_scatter_chunkcnt(abd);
+	for (int i = 0; i < n; i++) {
+		abd_free_chunk(ABD_SCATTER(abd).abd_chunks[i]);
+	}
+}
+
+abd_t *
+abd_alloc_struct(size_t size)
+{
+	size_t chunkcnt = abd_chunkcnt_for_bytes(size);
+	size_t abd_size = offsetof(abd_t,
+	    abd_u.abd_scatter.abd_chunks[chunkcnt]);
+	abd_t *abd = kmem_alloc(abd_size, KM_PUSHPAGE);
+	ASSERT3P(abd, !=, NULL);
+	ABDSTAT_INCR(abdstat_struct_size, abd_size);
+
+	return (abd);
+}
+
+void
+abd_free_struct(abd_t *abd)
+{
+	size_t chunkcnt = abd_is_linear(abd) ? 0 : abd_scatter_chunkcnt(abd);
+	int size = offsetof(abd_t, abd_u.abd_scatter.abd_chunks[chunkcnt]);
+	kmem_free(abd, size);
+	ABDSTAT_INCR(abdstat_struct_size, -size);
+}
+
+void
+abd_init(void)
+{
+	abd_chunk_cache = kmem_cache_create("abd_chunk", zfs_abd_chunk_size, 0,
+	    NULL, NULL, NULL, NULL, 0, KMC_NOTOUCH | KMC_NODEBUG);
+
+	abd_ksp = kstat_create("zfs", 0, "abdstats", "misc", KSTAT_TYPE_NAMED,
+	    sizeof (abd_stats) / sizeof (kstat_named_t), KSTAT_FLAG_VIRTUAL);
+	if (abd_ksp != NULL) {
+		abd_ksp->ks_data = &abd_stats;
+		kstat_install(abd_ksp);
+	}
+}
+
+void
+abd_fini(void)
+{
+	if (abd_ksp != NULL) {
+		kstat_delete(abd_ksp);
+		abd_ksp = NULL;
+	}
+
+	kmem_cache_destroy(abd_chunk_cache);
+	abd_chunk_cache = NULL;
+}
+
+void
+abd_free_linear_page(abd_t *abd)
+{
+	/*
+	 * FreeBSD does not have have scatter linear pages
+	 * so there is an error.
+	 */
+	VERIFY(0);
+}
+
+/*
+ * If we're going to use this ABD for doing I/O using the block layer, the
+ * consumer of the ABD data doesn't care if it's scattered or not, and we don't
+ * plan to store this ABD in memory for a long period of time, we should
+ * allocate the ABD type that requires the least data copying to do the I/O.
+ *
+ * Currently this is linear ABDs, however if ldi_strategy() can ever issue I/Os
+ * using a scatter/gather list we should switch to that and replace this call
+ * with vanilla abd_alloc().
+ */
+abd_t *
+abd_alloc_for_io(size_t size, boolean_t is_metadata)
+{
+	return (abd_alloc_linear(size, is_metadata));
+}
+
+/*
+ * This is just a helper function to abd_get_offset_scatter() to alloc a
+ * scatter ABD using the calculated chunkcnt based on the offset within the
+ * parent ABD.
+ */
+static abd_t *
+abd_alloc_scatter_offset_chunkcnt(size_t chunkcnt)
+{
+	size_t abd_size = offsetof(abd_t,
+	    abd_u.abd_scatter.abd_chunks[chunkcnt]);
+	abd_t *abd = kmem_alloc(abd_size, KM_PUSHPAGE);
+	ASSERT3P(abd, !=, NULL);
+	ABDSTAT_INCR(abdstat_struct_size, abd_size);
+
+	return (abd);
+}
+
+
+abd_t *
+abd_get_offset_scatter(abd_t *sabd, size_t off)
+{
+	abd_t *abd = NULL;
+
+	abd_verify(sabd);
+	ASSERT3U(off, <=, sabd->abd_size);
+
+	size_t new_offset = ABD_SCATTER(sabd).abd_offset + off;
+	size_t chunkcnt = abd_scatter_chunkcnt(sabd) -
+	    (new_offset / zfs_abd_chunk_size);
+
+	abd = abd_alloc_scatter_offset_chunkcnt(chunkcnt);
+
+	/*
+	 * Even if this buf is filesystem metadata, we only track that
+	 * if we own the underlying data buffer, which is not true in
+	 * this case. Therefore, we don't ever use ABD_FLAG_META here.
+	 */
+	abd->abd_flags = 0;
+
+	ABD_SCATTER(abd).abd_offset = new_offset % zfs_abd_chunk_size;
+	ABD_SCATTER(abd).abd_chunk_size = zfs_abd_chunk_size;
+
+	/* Copy the scatterlist starting at the correct offset */
+	(void) memcpy(&ABD_SCATTER(abd).abd_chunks,
+	    &ABD_SCATTER(sabd).abd_chunks[new_offset /
+	    zfs_abd_chunk_size],
+	    chunkcnt * sizeof (void *));
+
+	return (abd);
+}
+
+static inline size_t
+abd_iter_scatter_chunk_offset(struct abd_iter *aiter)
+{
+	ASSERT(!abd_is_linear(aiter->iter_abd));
+	return ((ABD_SCATTER(aiter->iter_abd).abd_offset +
+	    aiter->iter_pos) % zfs_abd_chunk_size);
+}
+
+static inline size_t
+abd_iter_scatter_chunk_index(struct abd_iter *aiter)
+{
+	ASSERT(!abd_is_linear(aiter->iter_abd));
+	return ((ABD_SCATTER(aiter->iter_abd).abd_offset +
+	    aiter->iter_pos) / zfs_abd_chunk_size);
+}
+
+/*
+ * Initialize the abd_iter.
+ */
+void
+abd_iter_init(struct abd_iter *aiter, abd_t *abd)
+{
+	abd_verify(abd);
+	aiter->iter_abd = abd;
+	aiter->iter_pos = 0;
+	aiter->iter_mapaddr = NULL;
+	aiter->iter_mapsize = 0;
+}
+
+/*
+ * This is just a helper function to see if we have exhausted the
+ * abd_iter and reached the end.
+ */
+boolean_t
+abd_iter_at_end(struct abd_iter *aiter)
+{
+	return (aiter->iter_pos == aiter->iter_abd->abd_size);
+}
+
+/*
+ * Advance the iterator by a certain amount. Cannot be called when a chunk is
+ * in use. This can be safely called when the aiter has already exhausted, in
+ * which case this does nothing.
+ */
+void
+abd_iter_advance(struct abd_iter *aiter, size_t amount)
+{
+	ASSERT3P(aiter->iter_mapaddr, ==, NULL);
+	ASSERT0(aiter->iter_mapsize);
+
+	/* There's nothing left to advance to, so do nothing */
+	if (abd_iter_at_end(aiter))
+		return;
+
+	aiter->iter_pos += amount;
+}
+
+/*
+ * Map the current chunk into aiter. This can be safely called when the aiter
+ * has already exhausted, in which case this does nothing.
+ */
+void
+abd_iter_map(struct abd_iter *aiter)
+{
+	void *paddr;
+	size_t offset = 0;
+
+	ASSERT3P(aiter->iter_mapaddr, ==, NULL);
+	ASSERT0(aiter->iter_mapsize);
+
+	/* Panic if someone has changed zfs_abd_chunk_size */
+	IMPLY(!abd_is_linear(aiter->iter_abd), zfs_abd_chunk_size ==
+	    ABD_SCATTER(aiter->iter_abd).abd_chunk_size);
+
+	/* There's nothing left to iterate over, so do nothing */
+	if (abd_iter_at_end(aiter))
+		return;
+
+	if (abd_is_linear(aiter->iter_abd)) {
+		offset = aiter->iter_pos;
+		aiter->iter_mapsize = aiter->iter_abd->abd_size - offset;
+		paddr = ABD_LINEAR_BUF(aiter->iter_abd);
+	} else {
+		size_t index = abd_iter_scatter_chunk_index(aiter);
+		offset = abd_iter_scatter_chunk_offset(aiter);
+		aiter->iter_mapsize = MIN(zfs_abd_chunk_size - offset,
+		    aiter->iter_abd->abd_size - aiter->iter_pos);
+		paddr = ABD_SCATTER(aiter->iter_abd).abd_chunks[index];
+	}
+	aiter->iter_mapaddr = (char *)paddr + offset;
+}
+
+/*
+ * Unmap the current chunk from aiter. This can be safely called when the aiter
+ * has already exhausted, in which case this does nothing.
+ */
+void
+abd_iter_unmap(struct abd_iter *aiter)
+{
+	/* There's nothing left to unmap, so do nothing */
+	if (abd_iter_at_end(aiter))
+		return;
+
+	ASSERT3P(aiter->iter_mapaddr, !=, NULL);
+	ASSERT3U(aiter->iter_mapsize, >, 0);
+
+	aiter->iter_mapaddr = NULL;
+	aiter->iter_mapsize = 0;
+}
+
+void
+abd_enter_critical(unsigned long flags)
+{
+	critical_enter();
+}
+
+void
+abd_exit_critical(unsigned long flags)
+{
+	critical_exit();
+}
diff --git a/module/os/linux/zfs/Makefile.in b/module/os/linux/zfs/Makefile.in
index 8c11a1ee6..cb4edbbc1 100644
--- a/module/os/linux/zfs/Makefile.in
+++ b/module/os/linux/zfs/Makefile.in
@@ -7,7 +7,7 @@ ccflags-$(CONFIG_SPARC64) += -Wno-unused-value
 
 ccflags-y += -I@abs_top_srcdir@/module/os/linux/zfs
 
-$(MODULE)-objs += ../os/linux/zfs/abd.o
+$(MODULE)-objs += ../os/linux/zfs/abd_os.o
 $(MODULE)-objs += ../os/linux/zfs/arc_os.o
 $(MODULE)-objs += ../os/linux/zfs/mmp_os.o
 $(MODULE)-objs += ../os/linux/zfs/policy.o
diff --git a/module/os/linux/zfs/abd.c b/module/os/linux/zfs/abd.c
deleted file mode 100644
index bc6f81000..000000000
--- a/module/os/linux/zfs/abd.c
+++ /dev/null
@@ -1,1616 +0,0 @@
-/*
- * CDDL HEADER START
- *
- * The contents of this file are subject to the terms of the
- * Common Development and Distribution License (the "License").
- * You may not use this file except in compliance with the License.
- *
- * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
- * or http://www.opensolaris.org/os/licensing.
- * See the License for the specific language governing permissions
- * and limitations under the License.
- *
- * When distributing Covered Code, include this CDDL HEADER in each
- * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
- * If applicable, add the following below this CDDL HEADER, with the
- * fields enclosed by brackets "[]" replaced with your own identifying
- * information: Portions Copyright [yyyy] [name of copyright owner]
- *
- * CDDL HEADER END
- */
-/*
- * Copyright (c) 2014 by Chunwei Chen. All rights reserved.
- * Copyright (c) 2019 by Delphix. All rights reserved.
- */
-
-/*
- * ARC buffer data (ABD).
- *
- * ABDs are an abstract data structure for the ARC which can use two
- * different ways of storing the underlying data:
- *
- * (a) Linear buffer. In this case, all the data in the ABD is stored in one
- *     contiguous buffer in memory (from a zio_[data_]buf_* kmem cache).
- *
- *         +-------------------+
- *         | ABD (linear)      |
- *         |   abd_flags = ... |
- *         |   abd_size = ...  |     +--------------------------------+
- *         |   abd_buf ------------->| raw buffer of size abd_size    |
- *         +-------------------+     +--------------------------------+
- *              no abd_chunks
- *
- * (b) Scattered buffer. In this case, the data in the ABD is split into
- *     equal-sized chunks (from the abd_chunk_cache kmem_cache), with pointers
- *     to the chunks recorded in an array at the end of the ABD structure.
- *
- *         +-------------------+
- *         | ABD (scattered)   |
- *         |   abd_flags = ... |
- *         |   abd_size = ...  |
- *         |   abd_offset = 0  |                           +-----------+
- *         |   abd_chunks[0] ----------------------------->| chunk 0   |
- *         |   abd_chunks[1] ---------------------+        +-----------+
- *         |   ...             |                  |        +-----------+
- *         |   abd_chunks[N-1] ---------+         +------->| chunk 1   |
- *         +-------------------+        |                  +-----------+
- *                                      |                      ...
- *                                      |                  +-----------+
- *                                      +----------------->| chunk N-1 |
- *                                                         +-----------+
- *
- * Linear buffers act exactly like normal buffers and are always mapped into the
- * kernel's virtual memory space, while scattered ABD data chunks are allocated
- * as physical pages and then mapped in only while they are actually being
- * accessed through one of the abd_* library functions. Using scattered ABDs
- * provides several benefits:
- *
- *  (1) They avoid use of kmem_*, preventing performance problems where running
- *      kmem_reap on very large memory systems never finishes and causes
- *      constant TLB shootdowns.
- *
- *  (2) Fragmentation is less of an issue since when we are at the limit of
- *      allocatable space, we won't have to search around for a long free
- *      hole in the VA space for large ARC allocations. Each chunk is mapped in
- *      individually, so even if we are using HIGHMEM (see next point) we
- *      wouldn't need to worry about finding a contiguous address range.
- *
- *  (3) If we are not using HIGHMEM, then all physical memory is always
- *      mapped into the kernel's address space, so we also avoid the map /
- *      unmap costs on each ABD access.
- *
- * If we are not using HIGHMEM, scattered buffers which have only one chunk
- * can be treated as linear buffers, because they are contiguous in the
- * kernel's virtual address space.  See abd_alloc_pages() for details.
- *
- * It is possible to make all ABDs linear by setting zfs_abd_scatter_enabled to
- * B_FALSE.
- *
- * In addition to directly allocating a linear or scattered ABD, it is also
- * possible to create an ABD by requesting the "sub-ABD" starting at an offset
- * within an existing ABD. In linear buffers this is simple (set abd_buf of
- * the new ABD to the starting point within the original raw buffer), but
- * scattered ABDs are a little more complex. The new ABD makes a copy of the
- * relevant abd_chunks pointers (but not the underlying data). However, to
- * provide arbitrary rather than only chunk-aligned starting offsets, it also
- * tracks an abd_offset field which represents the starting point of the data
- * within the first chunk in abd_chunks. For both linear and scattered ABDs,
- * creating an offset ABD marks the original ABD as the offset's parent, and the
- * original ABD's abd_children refcount is incremented. This data allows us to
- * ensure the root ABD isn't deleted before its children.
- *
- * Most consumers should never need to know what type of ABD they're using --
- * the ABD public API ensures that it's possible to transparently switch from
- * using a linear ABD to a scattered one when doing so would be beneficial.
- *
- * If you need to use the data within an ABD directly, if you know it's linear
- * (because you allocated it) you can use abd_to_buf() to access the underlying
- * raw buffer. Otherwise, you should use one of the abd_borrow_buf* functions
- * which will allocate a raw buffer if necessary. Use the abd_return_buf*
- * functions to return any raw buffers that are no longer necessary when you're
- * done using them.
- *
- * There are a variety of ABD APIs that implement basic buffer operations:
- * compare, copy, read, write, and fill with zeroes. If you need a custom
- * function which progressively accesses the whole ABD, use the abd_iterate_*
- * functions.
- */
-
-#include <sys/abd.h>
-#include <sys/param.h>
-#include <sys/zio.h>
-#include <sys/zfs_context.h>
-#include <sys/zfs_znode.h>
-#ifdef _KERNEL
-#include <linux/scatterlist.h>
-#include <linux/kmap_compat.h>
-#else
-#define	MAX_ORDER	1
-#endif
-
-typedef struct abd_stats {
-	kstat_named_t abdstat_struct_size;
-	kstat_named_t abdstat_linear_cnt;
-	kstat_named_t abdstat_linear_data_size;
-	kstat_named_t abdstat_scatter_cnt;
-	kstat_named_t abdstat_scatter_data_size;
-	kstat_named_t abdstat_scatter_chunk_waste;
-	kstat_named_t abdstat_scatter_orders[MAX_ORDER];
-	kstat_named_t abdstat_scatter_page_multi_chunk;
-	kstat_named_t abdstat_scatter_page_multi_zone;
-	kstat_named_t abdstat_scatter_page_alloc_retry;
-	kstat_named_t abdstat_scatter_sg_table_retry;
-} abd_stats_t;
-
-static abd_stats_t abd_stats = {
-	/* Amount of memory occupied by all of the abd_t struct allocations */
-	{ "struct_size",			KSTAT_DATA_UINT64 },
-	/*
-	 * The number of linear ABDs which are currently allocated, excluding
-	 * ABDs which don't own their data (for instance the ones which were
-	 * allocated through abd_get_offset() and abd_get_from_buf()). If an
-	 * ABD takes ownership of its buf then it will become tracked.
-	 */
-	{ "linear_cnt",				KSTAT_DATA_UINT64 },
-	/* Amount of data stored in all linear ABDs tracked by linear_cnt */
-	{ "linear_data_size",			KSTAT_DATA_UINT64 },
-	/*
-	 * The number of scatter ABDs which are currently allocated, excluding
-	 * ABDs which don't own their data (for instance the ones which were
-	 * allocated through abd_get_offset()).
-	 */
-	{ "scatter_cnt",			KSTAT_DATA_UINT64 },
-	/* Amount of data stored in all scatter ABDs tracked by scatter_cnt */
-	{ "scatter_data_size",			KSTAT_DATA_UINT64 },
-	/*
-	 * The amount of space wasted at the end of the last chunk across all
-	 * scatter ABDs tracked by scatter_cnt.
-	 */
-	{ "scatter_chunk_waste",		KSTAT_DATA_UINT64 },
-	/*
-	 * The number of compound allocations of a given order.  These
-	 * allocations are spread over all currently allocated ABDs, and
-	 * act as a measure of memory fragmentation.
-	 */
-	{ { "scatter_order_N",			KSTAT_DATA_UINT64 } },
-	/*
-	 * The number of scatter ABDs which contain multiple chunks.
-	 * ABDs are preferentially allocated from the minimum number of
-	 * contiguous multi-page chunks, a single chunk is optimal.
-	 */
-	{ "scatter_page_multi_chunk",		KSTAT_DATA_UINT64 },
-	/*
-	 * The number of scatter ABDs which are split across memory zones.
-	 * ABDs are preferentially allocated using pages from a single zone.
-	 */
-	{ "scatter_page_multi_zone",		KSTAT_DATA_UINT64 },
-	/*
-	 *  The total number of retries encountered when attempting to
-	 *  allocate the pages to populate the scatter ABD.
-	 */
-	{ "scatter_page_alloc_retry",		KSTAT_DATA_UINT64 },
-	/*
-	 *  The total number of retries encountered when attempting to
-	 *  allocate the sg table for an ABD.
-	 */
-	{ "scatter_sg_table_retry",		KSTAT_DATA_UINT64 },
-};
-
-#define	ABDSTAT(stat)		(abd_stats.stat.value.ui64)
-#define	ABDSTAT_INCR(stat, val) \
-	atomic_add_64(&abd_stats.stat.value.ui64, (val))
-#define	ABDSTAT_BUMP(stat)	ABDSTAT_INCR(stat, 1)
-#define	ABDSTAT_BUMPDOWN(stat)	ABDSTAT_INCR(stat, -1)
-
-#define	ABD_SCATTER(abd)	(abd->abd_u.abd_scatter)
-#define	ABD_BUF(abd)		(abd->abd_u.abd_linear.abd_buf)
-#define	abd_for_each_sg(abd, sg, n, i)	\
-	for_each_sg(ABD_SCATTER(abd).abd_sgl, sg, n, i)
-
-/* see block comment above for description */
-int zfs_abd_scatter_enabled = B_TRUE;
-unsigned zfs_abd_scatter_max_order = MAX_ORDER - 1;
-
-/*
- * zfs_abd_scatter_min_size is the minimum allocation size to use scatter
- * ABD's.  Smaller allocations will use linear ABD's which uses
- * zio_[data_]buf_alloc().
- *
- * Scatter ABD's use at least one page each, so sub-page allocations waste
- * some space when allocated as scatter (e.g. 2KB scatter allocation wastes
- * half of each page).  Using linear ABD's for small allocations means that
- * they will be put on slabs which contain many allocations.  This can
- * improve memory efficiency, but it also makes it much harder for ARC
- * evictions to actually free pages, because all the buffers on one slab need
- * to be freed in order for the slab (and underlying pages) to be freed.
- * Typically, 512B and 1KB kmem caches have 16 buffers per slab, so it's
- * possible for them to actually waste more memory than scatter (one page per
- * buf = wasting 3/4 or 7/8th; one buf per slab = wasting 15/16th).
- *
- * Spill blocks are typically 512B and are heavily used on systems running
- * selinux with the default dnode size and the `xattr=sa` property set.
- *
- * By default we use linear allocations for 512B and 1KB, and scatter
- * allocations for larger (1.5KB and up).
- */
-int zfs_abd_scatter_min_size = 512 * 3;
-
-static kmem_cache_t *abd_cache = NULL;
-static kstat_t *abd_ksp;
-
-static inline size_t
-abd_chunkcnt_for_bytes(size_t size)
-{
-	return (P2ROUNDUP(size, PAGESIZE) / PAGESIZE);
-}
-
-#ifdef _KERNEL
-/*
- * Mark zfs data pages so they can be excluded from kernel crash dumps
- */
-#ifdef _LP64
-#define	ABD_FILE_CACHE_PAGE	0x2F5ABDF11ECAC4E
-
-static inline void
-abd_mark_zfs_page(struct page *page)
-{
-	get_page(page);
-	SetPagePrivate(page);
-	set_page_private(page, ABD_FILE_CACHE_PAGE);
-}
-
-static inline void
-abd_unmark_zfs_page(struct page *page)
-{
-	set_page_private(page, 0UL);
-	ClearPagePrivate(page);
-	put_page(page);
-}
-#else
-#define	abd_mark_zfs_page(page)
-#define	abd_unmark_zfs_page(page)
-#endif /* _LP64 */
-
-#ifndef CONFIG_HIGHMEM
-
-#ifndef __GFP_RECLAIM
-#define	__GFP_RECLAIM		__GFP_WAIT
-#endif
-
-/*
- * The goal is to minimize fragmentation by preferentially populating ABDs
- * with higher order compound pages from a single zone.  Allocation size is
- * progressively decreased until it can be satisfied without performing
- * reclaim or compaction.  When necessary this function will degenerate to
- * allocating individual pages and allowing reclaim to satisfy allocations.
- */
-static void
-abd_alloc_pages(abd_t *abd, size_t size)
-{
-	struct list_head pages;
-	struct sg_table table;
-	struct scatterlist *sg;
-	struct page *page, *tmp_page = NULL;
-	gfp_t gfp = __GFP_NOWARN | GFP_NOIO;
-	gfp_t gfp_comp = (gfp | __GFP_NORETRY | __GFP_COMP) & ~__GFP_RECLAIM;
-	int max_order = MIN(zfs_abd_scatter_max_order, MAX_ORDER - 1);
-	int nr_pages = abd_chunkcnt_for_bytes(size);
-	int chunks = 0, zones = 0;
-	size_t remaining_size;
-	int nid = NUMA_NO_NODE;
-	int alloc_pages = 0;
-
-	INIT_LIST_HEAD(&pages);
-
-	while (alloc_pages < nr_pages) {
-		unsigned chunk_pages;
-		int order;
-
-		order = MIN(highbit64(nr_pages - alloc_pages) - 1, max_order);
-		chunk_pages = (1U << order);
-
-		page = alloc_pages_node(nid, order ? gfp_comp : gfp, order);
-		if (page == NULL) {
-			if (order == 0) {
-				ABDSTAT_BUMP(abdstat_scatter_page_alloc_retry);
-				schedule_timeout_interruptible(1);
-			} else {
-				max_order = MAX(0, order - 1);
-			}
-			continue;
-		}
-
-		list_add_tail(&page->lru, &pages);
-
-		if ((nid != NUMA_NO_NODE) && (page_to_nid(page) != nid))
-			zones++;
-
-		nid = page_to_nid(page);
-		ABDSTAT_BUMP(abdstat_scatter_orders[order]);
-		chunks++;
-		alloc_pages += chunk_pages;
-	}
-
-	ASSERT3S(alloc_pages, ==, nr_pages);
-
-	while (sg_alloc_table(&table, chunks, gfp)) {
-		ABDSTAT_BUMP(abdstat_scatter_sg_table_retry);
-		schedule_timeout_interruptible(1);
-	}
-
-	sg = table.sgl;
-	remaining_size = size;
-	list_for_each_entry_safe(page, tmp_page, &pages, lru) {
-		size_t sg_size = MIN(PAGESIZE << compound_order(page),
-		    remaining_size);
-		sg_set_page(sg, page, sg_size, 0);
-		abd_mark_zfs_page(page);
-		remaining_size -= sg_size;
-
-		sg = sg_next(sg);
-		list_del(&page->lru);
-	}
-
-	/*
-	 * These conditions ensure that a possible transformation to a linear
-	 * ABD would be valid.
-	 */
-	ASSERT(!PageHighMem(sg_page(table.sgl)));
-	ASSERT0(ABD_SCATTER(abd).abd_offset);
-
-	if (table.nents == 1) {
-		/*
-		 * Since there is only one entry, this ABD can be represented
-		 * as a linear buffer.  All single-page (4K) ABD's can be
-		 * represented this way.  Some multi-page ABD's can also be
-		 * represented this way, if we were able to allocate a single
-		 * "chunk" (higher-order "page" which represents a power-of-2
-		 * series of physically-contiguous pages).  This is often the
-		 * case for 2-page (8K) ABD's.
-		 *
-		 * Representing a single-entry scatter ABD as a linear ABD
-		 * has the performance advantage of avoiding the copy (and
-		 * allocation) in abd_borrow_buf_copy / abd_return_buf_copy.
-		 * A performance increase of around 5% has been observed for
-		 * ARC-cached reads (of small blocks which can take advantage
-		 * of this).
-		 *
-		 * Note that this optimization is only possible because the
-		 * pages are always mapped into the kernel's address space.
-		 * This is not the case for highmem pages, so the
-		 * optimization can not be made there.
-		 */
-		abd->abd_flags |= ABD_FLAG_LINEAR;
-		abd->abd_flags |= ABD_FLAG_LINEAR_PAGE;
-		abd->abd_u.abd_linear.abd_sgl = table.sgl;
-		abd->abd_u.abd_linear.abd_buf =
-		    page_address(sg_page(table.sgl));
-	} else if (table.nents > 1) {
-		ABDSTAT_BUMP(abdstat_scatter_page_multi_chunk);
-		abd->abd_flags |= ABD_FLAG_MULTI_CHUNK;
-
-		if (zones) {
-			ABDSTAT_BUMP(abdstat_scatter_page_multi_zone);
-			abd->abd_flags |= ABD_FLAG_MULTI_ZONE;
-		}
-
-		ABD_SCATTER(abd).abd_sgl = table.sgl;
-		ABD_SCATTER(abd).abd_nents = table.nents;
-	}
-}
-#else
-/*
- * Allocate N individual pages to construct a scatter ABD.  This function
- * makes no attempt to request contiguous pages and requires the minimal
- * number of kernel interfaces.  It's designed for maximum compatibility.
- */
-static void
-abd_alloc_pages(abd_t *abd, size_t size)
-{
-	struct scatterlist *sg = NULL;
-	struct sg_table table;
-	struct page *page;
-	gfp_t gfp = __GFP_NOWARN | GFP_NOIO;
-	int nr_pages = abd_chunkcnt_for_bytes(size);
-	int i = 0;
-
-	while (sg_alloc_table(&table, nr_pages, gfp)) {
-		ABDSTAT_BUMP(abdstat_scatter_sg_table_retry);
-		schedule_timeout_interruptible(1);
-	}
-
-	ASSERT3U(table.nents, ==, nr_pages);
-	ABD_SCATTER(abd).abd_sgl = table.sgl;
-	ABD_SCATTER(abd).abd_nents = nr_pages;
-
-	abd_for_each_sg(abd, sg, nr_pages, i) {
-		while ((page = __page_cache_alloc(gfp)) == NULL) {
-			ABDSTAT_BUMP(abdstat_scatter_page_alloc_retry);
-			schedule_timeout_interruptible(1);
-		}
-
-		ABDSTAT_BUMP(abdstat_scatter_orders[0]);
-		sg_set_page(sg, page, PAGESIZE, 0);
-		abd_mark_zfs_page(page);
-	}
-
-	if (nr_pages > 1) {
-		ABDSTAT_BUMP(abdstat_scatter_page_multi_chunk);
-		abd->abd_flags |= ABD_FLAG_MULTI_CHUNK;
-	}
-}
-#endif /* !CONFIG_HIGHMEM */
-
-static void
-abd_free_pages(abd_t *abd)
-{
-	struct scatterlist *sg = NULL;
-	struct sg_table table;
-	struct page *page;
-	int nr_pages = ABD_SCATTER(abd).abd_nents;
-	int order, i = 0;
-
-	if (abd->abd_flags & ABD_FLAG_MULTI_ZONE)
-		ABDSTAT_BUMPDOWN(abdstat_scatter_page_multi_zone);
-
-	if (abd->abd_flags & ABD_FLAG_MULTI_CHUNK)
-		ABDSTAT_BUMPDOWN(abdstat_scatter_page_multi_chunk);
-
-	abd_for_each_sg(abd, sg, nr_pages, i) {
-		page = sg_page(sg);
-		abd_unmark_zfs_page(page);
-		order = compound_order(page);
-		__free_pages(page, order);
-		ASSERT3U(sg->length, <=, PAGE_SIZE << order);
-		ABDSTAT_BUMPDOWN(abdstat_scatter_orders[order]);
-	}
-
-	table.sgl = ABD_SCATTER(abd).abd_sgl;
-	table.nents = table.orig_nents = nr_pages;
-	sg_free_table(&table);
-}
-
-#else /* _KERNEL */
-
-#ifndef PAGE_SHIFT
-#define	PAGE_SHIFT (highbit64(PAGESIZE)-1)
-#endif
-
-struct page;
-
-#define	zfs_kmap_atomic(chunk, km)	((void *)chunk)
-#define	zfs_kunmap_atomic(addr, km)	do { (void)(addr); } while (0)
-#define	local_irq_save(flags)		do { (void)(flags); } while (0)
-#define	local_irq_restore(flags)	do { (void)(flags); } while (0)
-#define	nth_page(pg, i) \
-	((struct page *)((void *)(pg) + (i) * PAGESIZE))
-
-struct scatterlist {
-	struct page *page;
-	int length;
-	int end;
-};
-
-static void
-sg_init_table(struct scatterlist *sg, int nr)
-{
-	memset(sg, 0, nr * sizeof (struct scatterlist));
-	sg[nr - 1].end = 1;
-}
-
-#define	for_each_sg(sgl, sg, nr, i)	\
-	for ((i) = 0, (sg) = (sgl); (i) < (nr); (i)++, (sg) = sg_next(sg))
-
-static inline void
-sg_set_page(struct scatterlist *sg, struct page *page, unsigned int len,
-    unsigned int offset)
-{
-	/* currently we don't use offset */
-	ASSERT(offset == 0);
-	sg->page = page;
-	sg->length = len;
-}
-
-static inline struct page *
-sg_page(struct scatterlist *sg)
-{
-	return (sg->page);
-}
-
-static inline struct scatterlist *
-sg_next(struct scatterlist *sg)
-{
-	if (sg->end)
-		return (NULL);
-
-	return (sg + 1);
-}
-
-static void
-abd_alloc_pages(abd_t *abd, size_t size)
-{
-	unsigned nr_pages = abd_chunkcnt_for_bytes(size);
-	struct scatterlist *sg;
-	int i;
-
-	ABD_SCATTER(abd).abd_sgl = vmem_alloc(nr_pages *
-	    sizeof (struct scatterlist), KM_SLEEP);
-	sg_init_table(ABD_SCATTER(abd).abd_sgl, nr_pages);
-
-	abd_for_each_sg(abd, sg, nr_pages, i) {
-		struct page *p = umem_alloc_aligned(PAGESIZE, 64, KM_SLEEP);
-		sg_set_page(sg, p, PAGESIZE, 0);
-	}
-	ABD_SCATTER(abd).abd_nents = nr_pages;
-}
-
-static void
-abd_free_pages(abd_t *abd)
-{
-	int i, n = ABD_SCATTER(abd).abd_nents;
-	struct scatterlist *sg;
-
-	abd_for_each_sg(abd, sg, n, i) {
-		for (int j = 0; j < sg->length; j += PAGESIZE) {
-			struct page *p = nth_page(sg_page(sg), j >> PAGE_SHIFT);
-			umem_free(p, PAGESIZE);
-		}
-	}
-
-	vmem_free(ABD_SCATTER(abd).abd_sgl, n * sizeof (struct scatterlist));
-}
-
-#endif /* _KERNEL */
-
-void
-abd_init(void)
-{
-	int i;
-
-	abd_cache = kmem_cache_create("abd_t", sizeof (abd_t),
-	    0, NULL, NULL, NULL, NULL, NULL, 0);
-
-	abd_ksp = kstat_create("zfs", 0, "abdstats", "misc", KSTAT_TYPE_NAMED,
-	    sizeof (abd_stats) / sizeof (kstat_named_t), KSTAT_FLAG_VIRTUAL);
-	if (abd_ksp != NULL) {
-		for (i = 0; i < MAX_ORDER; i++) {
-			snprintf(abd_stats.abdstat_scatter_orders[i].name,
-			    KSTAT_STRLEN, "scatter_order_%d", i);
-			abd_stats.abdstat_scatter_orders[i].data_type =
-			    KSTAT_DATA_UINT64;
-		}
-		abd_ksp->ks_data = &abd_stats;
-		kstat_install(abd_ksp);
-	}
-}
-
-void
-abd_fini(void)
-{
-	if (abd_ksp != NULL) {
-		kstat_delete(abd_ksp);
-		abd_ksp = NULL;
-	}
-
-	if (abd_cache) {
-		kmem_cache_destroy(abd_cache);
-		abd_cache = NULL;
-	}
-}
-
-static inline void
-abd_verify(abd_t *abd)
-{
-	ASSERT3U(abd->abd_size, >, 0);
-	ASSERT3U(abd->abd_size, <=, SPA_MAXBLOCKSIZE);
-	ASSERT3U(abd->abd_flags, ==, abd->abd_flags & (ABD_FLAG_LINEAR |
-	    ABD_FLAG_OWNER | ABD_FLAG_META | ABD_FLAG_MULTI_ZONE |
-	    ABD_FLAG_MULTI_CHUNK | ABD_FLAG_LINEAR_PAGE));
-	IMPLY(abd->abd_parent != NULL, !(abd->abd_flags & ABD_FLAG_OWNER));
-	IMPLY(abd->abd_flags & ABD_FLAG_META, abd->abd_flags & ABD_FLAG_OWNER);
-	if (abd_is_linear(abd)) {
-		ASSERT3P(abd->abd_u.abd_linear.abd_buf, !=, NULL);
-	} else {
-		size_t n;
-		int i = 0;
-		struct scatterlist *sg = NULL;
-
-		ASSERT3U(ABD_SCATTER(abd).abd_nents, >, 0);
-		ASSERT3U(ABD_SCATTER(abd).abd_offset, <,
-		    ABD_SCATTER(abd).abd_sgl->length);
-		n = ABD_SCATTER(abd).abd_nents;
-		abd_for_each_sg(abd, sg, n, i) {
-			ASSERT3P(sg_page(sg), !=, NULL);
-		}
-	}
-}
-
-static inline abd_t *
-abd_alloc_struct(void)
-{
-	abd_t *abd = kmem_cache_alloc(abd_cache, KM_PUSHPAGE);
-
-	ASSERT3P(abd, !=, NULL);
-	ABDSTAT_INCR(abdstat_struct_size, sizeof (abd_t));
-
-	return (abd);
-}
-
-static inline void
-abd_free_struct(abd_t *abd)
-{
-	kmem_cache_free(abd_cache, abd);
-	ABDSTAT_INCR(abdstat_struct_size, -(int)sizeof (abd_t));
-}
-
-/*
- * Allocate an ABD, along with its own underlying data buffers. Use this if you
- * don't care whether the ABD is linear or not.
- */
-abd_t *
-abd_alloc(size_t size, boolean_t is_metadata)
-{
-	/* see the comment above zfs_abd_scatter_min_size */
-	if (!zfs_abd_scatter_enabled || size < zfs_abd_scatter_min_size)
-		return (abd_alloc_linear(size, is_metadata));
-
-	VERIFY3U(size, <=, SPA_MAXBLOCKSIZE);
-
-	abd_t *abd = abd_alloc_struct();
-	abd->abd_flags = ABD_FLAG_OWNER;
-	abd->abd_u.abd_scatter.abd_offset = 0;
-	abd_alloc_pages(abd, size);
-
-	if (is_metadata) {
-		abd->abd_flags |= ABD_FLAG_META;
-	}
-	abd->abd_size = size;
-	abd->abd_parent = NULL;
-	zfs_refcount_create(&abd->abd_children);
-
-	ABDSTAT_BUMP(abdstat_scatter_cnt);
-	ABDSTAT_INCR(abdstat_scatter_data_size, size);
-	ABDSTAT_INCR(abdstat_scatter_chunk_waste,
-	    P2ROUNDUP(size, PAGESIZE) - size);
-
-	return (abd);
-}
-
-static void
-abd_free_scatter(abd_t *abd)
-{
-	abd_free_pages(abd);
-
-	zfs_refcount_destroy(&abd->abd_children);
-	ABDSTAT_BUMPDOWN(abdstat_scatter_cnt);
-	ABDSTAT_INCR(abdstat_scatter_data_size, -(int)abd->abd_size);
-	ABDSTAT_INCR(abdstat_scatter_chunk_waste,
-	    (int)abd->abd_size - (int)P2ROUNDUP(abd->abd_size, PAGESIZE));
-
-	abd_free_struct(abd);
-}
-
-/*
- * Allocate an ABD that must be linear, along with its own underlying data
- * buffer. Only use this when it would be very annoying to write your ABD
- * consumer with a scattered ABD.
- */
-abd_t *
-abd_alloc_linear(size_t size, boolean_t is_metadata)
-{
-	abd_t *abd = abd_alloc_struct();
-
-	VERIFY3U(size, <=, SPA_MAXBLOCKSIZE);
-
-	abd->abd_flags = ABD_FLAG_LINEAR | ABD_FLAG_OWNER;
-	if (is_metadata) {
-		abd->abd_flags |= ABD_FLAG_META;
-	}
-	abd->abd_size = size;
-	abd->abd_parent = NULL;
-	zfs_refcount_create(&abd->abd_children);
-
-	if (is_metadata) {
-		abd->abd_u.abd_linear.abd_buf = zio_buf_alloc(size);
-	} else {
-		abd->abd_u.abd_linear.abd_buf = zio_data_buf_alloc(size);
-	}
-
-	ABDSTAT_BUMP(abdstat_linear_cnt);
-	ABDSTAT_INCR(abdstat_linear_data_size, size);
-
-	return (abd);
-}
-
-static void
-abd_free_linear(abd_t *abd)
-{
-	if (abd_is_linear_page(abd)) {
-		/* Transform it back into a scatter ABD for freeing */
-		struct scatterlist *sg = abd->abd_u.abd_linear.abd_sgl;
-		abd->abd_flags &= ~ABD_FLAG_LINEAR;
-		abd->abd_flags &= ~ABD_FLAG_LINEAR_PAGE;
-		ABD_SCATTER(abd).abd_nents = 1;
-		ABD_SCATTER(abd).abd_offset = 0;
-		ABD_SCATTER(abd).abd_sgl = sg;
-		abd_free_scatter(abd);
-		return;
-	}
-	if (abd->abd_flags & ABD_FLAG_META) {
-		zio_buf_free(abd->abd_u.abd_linear.abd_buf, abd->abd_size);
-	} else {
-		zio_data_buf_free(abd->abd_u.abd_linear.abd_buf, abd->abd_size);
-	}
-
-	zfs_refcount_destroy(&abd->abd_children);
-	ABDSTAT_BUMPDOWN(abdstat_linear_cnt);
-	ABDSTAT_INCR(abdstat_linear_data_size, -(int)abd->abd_size);
-
-	abd_free_struct(abd);
-}
-
-/*
- * Free an ABD. Only use this on ABDs allocated with abd_alloc() or
- * abd_alloc_linear().
- */
-void
-abd_free(abd_t *abd)
-{
-	abd_verify(abd);
-	ASSERT3P(abd->abd_parent, ==, NULL);
-	ASSERT(abd->abd_flags & ABD_FLAG_OWNER);
-	if (abd_is_linear(abd))
-		abd_free_linear(abd);
-	else
-		abd_free_scatter(abd);
-}
-
-/*
- * Allocate an ABD of the same format (same metadata flag, same scatterize
- * setting) as another ABD.
- */
-abd_t *
-abd_alloc_sametype(abd_t *sabd, size_t size)
-{
-	boolean_t is_metadata = (sabd->abd_flags & ABD_FLAG_META) != 0;
-	if (abd_is_linear(sabd) &&
-	    !abd_is_linear_page(sabd)) {
-		return (abd_alloc_linear(size, is_metadata));
-	} else {
-		return (abd_alloc(size, is_metadata));
-	}
-}
-
-/*
- * If we're going to use this ABD for doing I/O using the block layer, the
- * consumer of the ABD data doesn't care if it's scattered or not, and we don't
- * plan to store this ABD in memory for a long period of time, we should
- * allocate the ABD type that requires the least data copying to do the I/O.
- *
- * On Illumos this is linear ABDs, however if ldi_strategy() can ever issue I/Os
- * using a scatter/gather list we should switch to that and replace this call
- * with vanilla abd_alloc().
- *
- * On Linux the optimal thing to do would be to use abd_get_offset() and
- * construct a new ABD which shares the original pages thereby eliminating
- * the copy.  But for the moment a new linear ABD is allocated until this
- * performance optimization can be implemented.
- */
-abd_t *
-abd_alloc_for_io(size_t size, boolean_t is_metadata)
-{
-	return (abd_alloc(size, is_metadata));
-}
-
-/*
- * Allocate a new ABD to point to offset off of sabd. It shares the underlying
- * buffer data with sabd. Use abd_put() to free. sabd must not be freed while
- * any derived ABDs exist.
- */
-static inline abd_t *
-abd_get_offset_impl(abd_t *sabd, size_t off, size_t size)
-{
-	abd_t *abd;
-
-	abd_verify(sabd);
-	ASSERT3U(off, <=, sabd->abd_size);
-
-	if (abd_is_linear(sabd)) {
-		abd = abd_alloc_struct();
-
-		/*
-		 * Even if this buf is filesystem metadata, we only track that
-		 * if we own the underlying data buffer, which is not true in
-		 * this case. Therefore, we don't ever use ABD_FLAG_META here.
-		 */
-		abd->abd_flags = ABD_FLAG_LINEAR;
-
-		abd->abd_u.abd_linear.abd_buf =
-		    (char *)sabd->abd_u.abd_linear.abd_buf + off;
-	} else {
-		int i = 0;
-		struct scatterlist *sg = NULL;
-		size_t new_offset = sabd->abd_u.abd_scatter.abd_offset + off;
-
-		abd = abd_alloc_struct();
-
-		/*
-		 * Even if this buf is filesystem metadata, we only track that
-		 * if we own the underlying data buffer, which is not true in
-		 * this case. Therefore, we don't ever use ABD_FLAG_META here.
-		 */
-		abd->abd_flags = 0;
-
-		abd_for_each_sg(sabd, sg, ABD_SCATTER(sabd).abd_nents, i) {
-			if (new_offset < sg->length)
-				break;
-			new_offset -= sg->length;
-		}
-
-		ABD_SCATTER(abd).abd_sgl = sg;
-		ABD_SCATTER(abd).abd_offset = new_offset;
-		ABD_SCATTER(abd).abd_nents = ABD_SCATTER(sabd).abd_nents - i;
-	}
-
-	abd->abd_size = size;
-	abd->abd_parent = sabd;
-	zfs_refcount_create(&abd->abd_children);
-	(void) zfs_refcount_add_many(&sabd->abd_children, abd->abd_size, abd);
-
-	return (abd);
-}
-
-abd_t *
-abd_get_offset(abd_t *sabd, size_t off)
-{
-	size_t size = sabd->abd_size > off ? sabd->abd_size - off : 0;
-
-	VERIFY3U(size, >, 0);
-
-	return (abd_get_offset_impl(sabd, off, size));
-}
-
-abd_t *
-abd_get_offset_size(abd_t *sabd, size_t off, size_t size)
-{
-	ASSERT3U(off + size, <=, sabd->abd_size);
-
-	return (abd_get_offset_impl(sabd, off, size));
-}
-
-/*
- * Allocate a linear ABD structure for buf. You must free this with abd_put()
- * since the resulting ABD doesn't own its own buffer.
- */
-abd_t *
-abd_get_from_buf(void *buf, size_t size)
-{
-	abd_t *abd = abd_alloc_struct();
-
-	VERIFY3U(size, <=, SPA_MAXBLOCKSIZE);
-
-	/*
-	 * Even if this buf is filesystem metadata, we only track that if we
-	 * own the underlying data buffer, which is not true in this case.
-	 * Therefore, we don't ever use ABD_FLAG_META here.
-	 */
-	abd->abd_flags = ABD_FLAG_LINEAR;
-	abd->abd_size = size;
-	abd->abd_parent = NULL;
-	zfs_refcount_create(&abd->abd_children);
-
-	abd->abd_u.abd_linear.abd_buf = buf;
-
-	return (abd);
-}
-
-/*
- * Free an ABD allocated from abd_get_offset() or abd_get_from_buf(). Will not
- * free the underlying scatterlist or buffer.
- */
-void
-abd_put(abd_t *abd)
-{
-	abd_verify(abd);
-	ASSERT(!(abd->abd_flags & ABD_FLAG_OWNER));
-
-	if (abd->abd_parent != NULL) {
-		(void) zfs_refcount_remove_many(&abd->abd_parent->abd_children,
-		    abd->abd_size, abd);
-	}
-
-	zfs_refcount_destroy(&abd->abd_children);
-	abd_free_struct(abd);
-}
-
-/*
- * Get the raw buffer associated with a linear ABD.
- */
-void *
-abd_to_buf(abd_t *abd)
-{
-	ASSERT(abd_is_linear(abd));
-	abd_verify(abd);
-	return (abd->abd_u.abd_linear.abd_buf);
-}
-
-/*
- * Borrow a raw buffer from an ABD without copying the contents of the ABD
- * into the buffer. If the ABD is scattered, this will allocate a raw buffer
- * whose contents are undefined. To copy over the existing data in the ABD, use
- * abd_borrow_buf_copy() instead.
- */
-void *
-abd_borrow_buf(abd_t *abd, size_t n)
-{
-	void *buf;
-	abd_verify(abd);
-	ASSERT3U(abd->abd_size, >=, n);
-	if (abd_is_linear(abd)) {
-		buf = abd_to_buf(abd);
-	} else {
-		buf = zio_buf_alloc(n);
-	}
-	(void) zfs_refcount_add_many(&abd->abd_children, n, buf);
-
-	return (buf);
-}
-
-void *
-abd_borrow_buf_copy(abd_t *abd, size_t n)
-{
-	void *buf = abd_borrow_buf(abd, n);
-	if (!abd_is_linear(abd)) {
-		abd_copy_to_buf(buf, abd, n);
-	}
-	return (buf);
-}
-
-/*
- * Return a borrowed raw buffer to an ABD. If the ABD is scattered, this will
- * not change the contents of the ABD and will ASSERT that you didn't modify
- * the buffer since it was borrowed. If you want any changes you made to buf to
- * be copied back to abd, use abd_return_buf_copy() instead.
- */
-void
-abd_return_buf(abd_t *abd, void *buf, size_t n)
-{
-	abd_verify(abd);
-	ASSERT3U(abd->abd_size, >=, n);
-	if (abd_is_linear(abd)) {
-		ASSERT3P(buf, ==, abd_to_buf(abd));
-	} else {
-		ASSERT0(abd_cmp_buf(abd, buf, n));
-		zio_buf_free(buf, n);
-	}
-	(void) zfs_refcount_remove_many(&abd->abd_children, n, buf);
-}
-
-void
-abd_return_buf_copy(abd_t *abd, void *buf, size_t n)
-{
-	if (!abd_is_linear(abd)) {
-		abd_copy_from_buf(abd, buf, n);
-	}
-	abd_return_buf(abd, buf, n);
-}
-
-/*
- * Give this ABD ownership of the buffer that it's storing. Can only be used on
- * linear ABDs which were allocated via abd_get_from_buf(), or ones allocated
- * with abd_alloc_linear() which subsequently released ownership of their buf
- * with abd_release_ownership_of_buf().
- */
-void
-abd_take_ownership_of_buf(abd_t *abd, boolean_t is_metadata)
-{
-	ASSERT(abd_is_linear(abd));
-	ASSERT(!(abd->abd_flags & ABD_FLAG_OWNER));
-	abd_verify(abd);
-
-	abd->abd_flags |= ABD_FLAG_OWNER;
-	if (is_metadata) {
-		abd->abd_flags |= ABD_FLAG_META;
-	}
-
-	ABDSTAT_BUMP(abdstat_linear_cnt);
-	ABDSTAT_INCR(abdstat_linear_data_size, abd->abd_size);
-}
-
-void
-abd_release_ownership_of_buf(abd_t *abd)
-{
-	ASSERT(abd_is_linear(abd));
-	ASSERT(abd->abd_flags & ABD_FLAG_OWNER);
-
-	/*
-	 * abd_free() needs to handle LINEAR_PAGE ABD's specially.
-	 * Since that flag does not survive the
-	 * abd_release_ownership_of_buf() -> abd_get_from_buf() ->
-	 * abd_take_ownership_of_buf() sequence, we don't allow releasing
-	 * these "linear but not zio_[data_]buf_alloc()'ed" ABD's.
-	 */
-	ASSERT(!abd_is_linear_page(abd));
-
-	abd_verify(abd);
-
-	abd->abd_flags &= ~ABD_FLAG_OWNER;
-	/* Disable this flag since we no longer own the data buffer */
-	abd->abd_flags &= ~ABD_FLAG_META;
-
-	ABDSTAT_BUMPDOWN(abdstat_linear_cnt);
-	ABDSTAT_INCR(abdstat_linear_data_size, -(int)abd->abd_size);
-}
-
-struct abd_iter {
-	/* public interface */
-	void		*iter_mapaddr;	/* addr corresponding to iter_pos */
-	size_t		iter_mapsize;	/* length of data valid at mapaddr */
-
-	/* private */
-	abd_t		*iter_abd;	/* ABD being iterated through */
-	size_t		iter_pos;
-	size_t		iter_offset;	/* offset in current sg/abd_buf, */
-					/* abd_offset included */
-	struct scatterlist *iter_sg;	/* current sg */
-};
-
-/*
- * Initialize the abd_iter.
- */
-static void
-abd_iter_init(struct abd_iter *aiter, abd_t *abd, int km_type)
-{
-	abd_verify(abd);
-	aiter->iter_abd = abd;
-	aiter->iter_mapaddr = NULL;
-	aiter->iter_mapsize = 0;
-	aiter->iter_pos = 0;
-	if (abd_is_linear(abd)) {
-		aiter->iter_offset = 0;
-		aiter->iter_sg = NULL;
-	} else {
-		aiter->iter_offset = ABD_SCATTER(abd).abd_offset;
-		aiter->iter_sg = ABD_SCATTER(abd).abd_sgl;
-	}
-}
-
-/*
- * Advance the iterator by a certain amount. Cannot be called when a chunk is
- * in use. This can be safely called when the aiter has already exhausted, in
- * which case this does nothing.
- */
-static void
-abd_iter_advance(struct abd_iter *aiter, size_t amount)
-{
-	ASSERT3P(aiter->iter_mapaddr, ==, NULL);
-	ASSERT0(aiter->iter_mapsize);
-
-	/* There's nothing left to advance to, so do nothing */
-	if (aiter->iter_pos == aiter->iter_abd->abd_size)
-		return;
-
-	aiter->iter_pos += amount;
-	aiter->iter_offset += amount;
-	if (!abd_is_linear(aiter->iter_abd)) {
-		while (aiter->iter_offset >= aiter->iter_sg->length) {
-			aiter->iter_offset -= aiter->iter_sg->length;
-			aiter->iter_sg = sg_next(aiter->iter_sg);
-			if (aiter->iter_sg == NULL) {
-				ASSERT0(aiter->iter_offset);
-				break;
-			}
-		}
-	}
-}
-
-/*
- * Map the current chunk into aiter. This can be safely called when the aiter
- * has already exhausted, in which case this does nothing.
- */
-static void
-abd_iter_map(struct abd_iter *aiter)
-{
-	void *paddr;
-	size_t offset = 0;
-
-	ASSERT3P(aiter->iter_mapaddr, ==, NULL);
-	ASSERT0(aiter->iter_mapsize);
-
-	/* There's nothing left to iterate over, so do nothing */
-	if (aiter->iter_pos == aiter->iter_abd->abd_size)
-		return;
-
-	if (abd_is_linear(aiter->iter_abd)) {
-		ASSERT3U(aiter->iter_pos, ==, aiter->iter_offset);
-		offset = aiter->iter_offset;
-		aiter->iter_mapsize = aiter->iter_abd->abd_size - offset;
-		paddr = aiter->iter_abd->abd_u.abd_linear.abd_buf;
-	} else {
-		offset = aiter->iter_offset;
-		aiter->iter_mapsize = MIN(aiter->iter_sg->length - offset,
-		    aiter->iter_abd->abd_size - aiter->iter_pos);
-
-		paddr = zfs_kmap_atomic(sg_page(aiter->iter_sg),
-		    km_table[aiter->iter_km]);
-	}
-
-	aiter->iter_mapaddr = (char *)paddr + offset;
-}
-
-/*
- * Unmap the current chunk from aiter. This can be safely called when the aiter
- * has already exhausted, in which case this does nothing.
- */
-static void
-abd_iter_unmap(struct abd_iter *aiter)
-{
-	/* There's nothing left to unmap, so do nothing */
-	if (aiter->iter_pos == aiter->iter_abd->abd_size)
-		return;
-
-	if (!abd_is_linear(aiter->iter_abd)) {
-		/* LINTED E_FUNC_SET_NOT_USED */
-		zfs_kunmap_atomic(aiter->iter_mapaddr - aiter->iter_offset,
-		    km_table[aiter->iter_km]);
-	}
-
-	ASSERT3P(aiter->iter_mapaddr, !=, NULL);
-	ASSERT3U(aiter->iter_mapsize, >, 0);
-
-	aiter->iter_mapaddr = NULL;
-	aiter->iter_mapsize = 0;
-}
-
-int
-abd_iterate_func(abd_t *abd, size_t off, size_t size,
-    abd_iter_func_t *func, void *private)
-{
-	int ret = 0;
-	struct abd_iter aiter;
-
-	abd_verify(abd);
-	ASSERT3U(off + size, <=, abd->abd_size);
-
-	abd_iter_init(&aiter, abd, 0);
-	abd_iter_advance(&aiter, off);
-
-	while (size > 0) {
-		abd_iter_map(&aiter);
-
-		size_t len = MIN(aiter.iter_mapsize, size);
-		ASSERT3U(len, >, 0);
-
-		ret = func(aiter.iter_mapaddr, len, private);
-
-		abd_iter_unmap(&aiter);
-
-		if (ret != 0)
-			break;
-
-		size -= len;
-		abd_iter_advance(&aiter, len);
-	}
-
-	return (ret);
-}
-
-struct buf_arg {
-	void *arg_buf;
-};
-
-static int
-abd_copy_to_buf_off_cb(void *buf, size_t size, void *private)
-{
-	struct buf_arg *ba_ptr = private;
-
-	(void) memcpy(ba_ptr->arg_buf, buf, size);
-	ba_ptr->arg_buf = (char *)ba_ptr->arg_buf + size;
-
-	return (0);
-}
-
-/*
- * Copy abd to buf. (off is the offset in abd.)
- */
-void
-abd_copy_to_buf_off(void *buf, abd_t *abd, size_t off, size_t size)
-{
-	struct buf_arg ba_ptr = { buf };
-
-	(void) abd_iterate_func(abd, off, size, abd_copy_to_buf_off_cb,
-	    &ba_ptr);
-}
-
-static int
-abd_cmp_buf_off_cb(void *buf, size_t size, void *private)
-{
-	int ret;
-	struct buf_arg *ba_ptr = private;
-
-	ret = memcmp(buf, ba_ptr->arg_buf, size);
-	ba_ptr->arg_buf = (char *)ba_ptr->arg_buf + size;
-
-	return (ret);
-}
-
-/*
- * Compare the contents of abd to buf. (off is the offset in abd.)
- */
-int
-abd_cmp_buf_off(abd_t *abd, const void *buf, size_t off, size_t size)
-{
-	struct buf_arg ba_ptr = { (void *) buf };
-
-	return (abd_iterate_func(abd, off, size, abd_cmp_buf_off_cb, &ba_ptr));
-}
-
-static int
-abd_copy_from_buf_off_cb(void *buf, size_t size, void *private)
-{
-	struct buf_arg *ba_ptr = private;
-
-	(void) memcpy(buf, ba_ptr->arg_buf, size);
-	ba_ptr->arg_buf = (char *)ba_ptr->arg_buf + size;
-
-	return (0);
-}
-
-/*
- * Copy from buf to abd. (off is the offset in abd.)
- */
-void
-abd_copy_from_buf_off(abd_t *abd, const void *buf, size_t off, size_t size)
-{
-	struct buf_arg ba_ptr = { (void *) buf };
-
-	(void) abd_iterate_func(abd, off, size, abd_copy_from_buf_off_cb,
-	    &ba_ptr);
-}
-
-/*ARGSUSED*/
-static int
-abd_zero_off_cb(void *buf, size_t size, void *private)
-{
-	(void) memset(buf, 0, size);
-	return (0);
-}
-
-/*
- * Zero out the abd from a particular offset to the end.
- */
-void
-abd_zero_off(abd_t *abd, size_t off, size_t size)
-{
-	(void) abd_iterate_func(abd, off, size, abd_zero_off_cb, NULL);
-}
-
-/*
- * Iterate over two ABDs and call func incrementally on the two ABDs' data in
- * equal-sized chunks (passed to func as raw buffers). func could be called many
- * times during this iteration.
- */
-int
-abd_iterate_func2(abd_t *dabd, abd_t *sabd, size_t doff, size_t soff,
-    size_t size, abd_iter_func2_t *func, void *private)
-{
-	int ret = 0;
-	struct abd_iter daiter, saiter;
-
-	abd_verify(dabd);
-	abd_verify(sabd);
-
-	ASSERT3U(doff + size, <=, dabd->abd_size);
-	ASSERT3U(soff + size, <=, sabd->abd_size);
-
-	abd_iter_init(&daiter, dabd, 0);
-	abd_iter_init(&saiter, sabd, 1);
-	abd_iter_advance(&daiter, doff);
-	abd_iter_advance(&saiter, soff);
-
-	while (size > 0) {
-		abd_iter_map(&daiter);
-		abd_iter_map(&saiter);
-
-		size_t dlen = MIN(daiter.iter_mapsize, size);
-		size_t slen = MIN(saiter.iter_mapsize, size);
-		size_t len = MIN(dlen, slen);
-		ASSERT(dlen > 0 || slen > 0);
-
-		ret = func(daiter.iter_mapaddr, saiter.iter_mapaddr, len,
-		    private);
-
-		abd_iter_unmap(&saiter);
-		abd_iter_unmap(&daiter);
-
-		if (ret != 0)
-			break;
-
-		size -= len;
-		abd_iter_advance(&daiter, len);
-		abd_iter_advance(&saiter, len);
-	}
-
-	return (ret);
-}
-
-/*ARGSUSED*/
-static int
-abd_copy_off_cb(void *dbuf, void *sbuf, size_t size, void *private)
-{
-	(void) memcpy(dbuf, sbuf, size);
-	return (0);
-}
-
-/*
- * Copy from sabd to dabd starting from soff and doff.
- */
-void
-abd_copy_off(abd_t *dabd, abd_t *sabd, size_t doff, size_t soff, size_t size)
-{
-	(void) abd_iterate_func2(dabd, sabd, doff, soff, size,
-	    abd_copy_off_cb, NULL);
-}
-
-/*ARGSUSED*/
-static int
-abd_cmp_cb(void *bufa, void *bufb, size_t size, void *private)
-{
-	return (memcmp(bufa, bufb, size));
-}
-
-/*
- * Compares the contents of two ABDs.
- */
-int
-abd_cmp(abd_t *dabd, abd_t *sabd)
-{
-	ASSERT3U(dabd->abd_size, ==, sabd->abd_size);
-	return (abd_iterate_func2(dabd, sabd, 0, 0, dabd->abd_size,
-	    abd_cmp_cb, NULL));
-}
-
-/*
- * Iterate over code ABDs and a data ABD and call @func_raidz_gen.
- *
- * @cabds          parity ABDs, must have equal size
- * @dabd           data ABD. Can be NULL (in this case @dsize = 0)
- * @func_raidz_gen should be implemented so that its behaviour
- *                 is the same when taking linear and when taking scatter
- */
-void
-abd_raidz_gen_iterate(abd_t **cabds, abd_t *dabd,
-    ssize_t csize, ssize_t dsize, const unsigned parity,
-    void (*func_raidz_gen)(void **, const void *, size_t, size_t))
-{
-	int i;
-	ssize_t len, dlen;
-	struct abd_iter caiters[3];
-	struct abd_iter daiter = {0};
-	void *caddrs[3];
-	unsigned long flags;
-
-	ASSERT3U(parity, <=, 3);
-
-	for (i = 0; i < parity; i++)
-		abd_iter_init(&caiters[i], cabds[i], i);
-
-	if (dabd)
-		abd_iter_init(&daiter, dabd, i);
-
-	ASSERT3S(dsize, >=, 0);
-
-	local_irq_save(flags);
-	while (csize > 0) {
-		len = csize;
-
-		if (dabd && dsize > 0)
-			abd_iter_map(&daiter);
-
-		for (i = 0; i < parity; i++) {
-			abd_iter_map(&caiters[i]);
-			caddrs[i] = caiters[i].iter_mapaddr;
-		}
-
-		switch (parity) {
-			case 3:
-				len = MIN(caiters[2].iter_mapsize, len);
-				/* falls through */
-			case 2:
-				len = MIN(caiters[1].iter_mapsize, len);
-				/* falls through */
-			case 1:
-				len = MIN(caiters[0].iter_mapsize, len);
-		}
-
-		/* must be progressive */
-		ASSERT3S(len, >, 0);
-
-		if (dabd && dsize > 0) {
-			/* this needs precise iter.length */
-			len = MIN(daiter.iter_mapsize, len);
-			dlen = len;
-		} else
-			dlen = 0;
-
-		/* must be progressive */
-		ASSERT3S(len, >, 0);
-		/*
-		 * The iterated function likely will not do well if each
-		 * segment except the last one is not multiple of 512 (raidz).
-		 */
-		ASSERT3U(((uint64_t)len & 511ULL), ==, 0);
-
-		func_raidz_gen(caddrs, daiter.iter_mapaddr, len, dlen);
-
-		for (i = parity-1; i >= 0; i--) {
-			abd_iter_unmap(&caiters[i]);
-			abd_iter_advance(&caiters[i], len);
-		}
-
-		if (dabd && dsize > 0) {
-			abd_iter_unmap(&daiter);
-			abd_iter_advance(&daiter, dlen);
-			dsize -= dlen;
-		}
-
-		csize -= len;
-
-		ASSERT3S(dsize, >=, 0);
-		ASSERT3S(csize, >=, 0);
-	}
-	local_irq_restore(flags);
-}
-
-/*
- * Iterate over code ABDs and data reconstruction target ABDs and call
- * @func_raidz_rec. Function maps at most 6 pages atomically.
- *
- * @cabds           parity ABDs, must have equal size
- * @tabds           rec target ABDs, at most 3
- * @tsize           size of data target columns
- * @func_raidz_rec  expects syndrome data in target columns. Function
- *                  reconstructs data and overwrites target columns.
- */
-void
-abd_raidz_rec_iterate(abd_t **cabds, abd_t **tabds,
-    ssize_t tsize, const unsigned parity,
-    void (*func_raidz_rec)(void **t, const size_t tsize, void **c,
-    const unsigned *mul),
-    const unsigned *mul)
-{
-	int i;
-	ssize_t len;
-	struct abd_iter citers[3];
-	struct abd_iter xiters[3];
-	void *caddrs[3], *xaddrs[3];
-	unsigned long flags;
-
-	ASSERT3U(parity, <=, 3);
-
-	for (i = 0; i < parity; i++) {
-		abd_iter_init(&citers[i], cabds[i], 2*i);
-		abd_iter_init(&xiters[i], tabds[i], 2*i+1);
-	}
-
-	local_irq_save(flags);
-	while (tsize > 0) {
-
-		for (i = 0; i < parity; i++) {
-			abd_iter_map(&citers[i]);
-			abd_iter_map(&xiters[i]);
-			caddrs[i] = citers[i].iter_mapaddr;
-			xaddrs[i] = xiters[i].iter_mapaddr;
-		}
-
-		len = tsize;
-		switch (parity) {
-			case 3:
-				len = MIN(xiters[2].iter_mapsize, len);
-				len = MIN(citers[2].iter_mapsize, len);
-				/* falls through */
-			case 2:
-				len = MIN(xiters[1].iter_mapsize, len);
-				len = MIN(citers[1].iter_mapsize, len);
-				/* falls through */
-			case 1:
-				len = MIN(xiters[0].iter_mapsize, len);
-				len = MIN(citers[0].iter_mapsize, len);
-		}
-		/* must be progressive */
-		ASSERT3S(len, >, 0);
-		/*
-		 * The iterated function likely will not do well if each
-		 * segment except the last one is not multiple of 512 (raidz).
-		 */
-		ASSERT3U(((uint64_t)len & 511ULL), ==, 0);
-
-		func_raidz_rec(xaddrs, len, caddrs, mul);
-
-		for (i = parity-1; i >= 0; i--) {
-			abd_iter_unmap(&xiters[i]);
-			abd_iter_unmap(&citers[i]);
-			abd_iter_advance(&xiters[i], len);
-			abd_iter_advance(&citers[i], len);
-		}
-
-		tsize -= len;
-		ASSERT3S(tsize, >=, 0);
-	}
-	local_irq_restore(flags);
-}
-
-#if defined(_KERNEL)
-/*
- * bio_nr_pages for ABD.
- * @off is the offset in @abd
- */
-unsigned long
-abd_nr_pages_off(abd_t *abd, unsigned int size, size_t off)
-{
-	unsigned long pos;
-
-	if (abd_is_linear(abd))
-		pos = (unsigned long)abd_to_buf(abd) + off;
-	else
-		pos = abd->abd_u.abd_scatter.abd_offset + off;
-
-	return ((pos + size + PAGESIZE - 1) >> PAGE_SHIFT) -
-	    (pos >> PAGE_SHIFT);
-}
-
-/*
- * bio_map for scatter ABD.
- * @off is the offset in @abd
- * Remaining IO size is returned
- */
-unsigned int
-abd_scatter_bio_map_off(struct bio *bio, abd_t *abd,
-    unsigned int io_size, size_t off)
-{
-	int i;
-	struct abd_iter aiter;
-
-	ASSERT(!abd_is_linear(abd));
-	ASSERT3U(io_size, <=, abd->abd_size - off);
-
-	abd_iter_init(&aiter, abd, 0);
-	abd_iter_advance(&aiter, off);
-
-	for (i = 0; i < bio->bi_max_vecs; i++) {
-		struct page *pg;
-		size_t len, sgoff, pgoff;
-		struct scatterlist *sg;
-
-		if (io_size <= 0)
-			break;
-
-		sg = aiter.iter_sg;
-		sgoff = aiter.iter_offset;
-		pgoff = sgoff & (PAGESIZE - 1);
-		len = MIN(io_size, PAGESIZE - pgoff);
-		ASSERT(len > 0);
-
-		pg = nth_page(sg_page(sg), sgoff >> PAGE_SHIFT);
-		if (bio_add_page(bio, pg, len, pgoff) != len)
-			break;
-
-		io_size -= len;
-		abd_iter_advance(&aiter, len);
-	}
-
-	return (io_size);
-}
-
-/* Tunable Parameters */
-module_param(zfs_abd_scatter_enabled, int, 0644);
-MODULE_PARM_DESC(zfs_abd_scatter_enabled,
-	"Toggle whether ABD allocations must be linear.");
-module_param(zfs_abd_scatter_min_size, int, 0644);
-MODULE_PARM_DESC(zfs_abd_scatter_min_size,
-	"Minimum size of scatter allocations.");
-/* CSTYLED */
-module_param(zfs_abd_scatter_max_order, uint, 0644);
-MODULE_PARM_DESC(zfs_abd_scatter_max_order,
-	"Maximum order allocation used for a scatter ABD.");
-#endif
diff --git a/module/os/linux/zfs/abd_os.c b/module/os/linux/zfs/abd_os.c
new file mode 100644
index 000000000..57e415ef3
--- /dev/null
+++ b/module/os/linux/zfs/abd_os.c
@@ -0,0 +1,891 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://www.opensolaris.org/os/licensing.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+/*
+ * Copyright (c) 2014 by Chunwei Chen. All rights reserved.
+ * Copyright (c) 2019 by Delphix. All rights reserved.
+ */
+
+/*
+ * See abd.c for an general overview of the arc buffered data (ABD).
+ *
+ * Linear buffers act exactly like normal buffers and are always mapped into the
+ * kernel's virtual memory space, while scattered ABD data chunks are allocated
+ * as physical pages and then mapped in only while they are actually being
+ * accessed through one of the abd_* library functions. Using scattered ABDs
+ * provides several benefits:
+ *
+ *  (1) They avoid use of kmem_*, preventing performance problems where running
+ *      kmem_reap on very large memory systems never finishes and causes
+ *      constant TLB shootdowns.
+ *
+ *  (2) Fragmentation is less of an issue since when we are at the limit of
+ *      allocatable space, we won't have to search around for a long free
+ *      hole in the VA space for large ARC allocations. Each chunk is mapped in
+ *      individually, so even if we are using HIGHMEM (see next point) we
+ *      wouldn't need to worry about finding a contiguous address range.
+ *
+ *  (3) If we are not using HIGHMEM, then all physical memory is always
+ *      mapped into the kernel's address space, so we also avoid the map /
+ *      unmap costs on each ABD access.
+ *
+ * If we are not using HIGHMEM, scattered buffers which have only one chunk
+ * can be treated as linear buffers, because they are contiguous in the
+ * kernel's virtual address space. See abd_alloc_chunks() for details.
+ */
+
+#include <sys/abd_impl.h>
+#include <sys/param.h>
+#include <sys/zio.h>
+#include <sys/zfs_context.h>
+#include <sys/zfs_znode.h>
+#ifdef _KERNEL
+#include <linux/kmap_compat.h>
+#include <linux/scatterlist.h>
+#else
+#define	MAX_ORDER	1
+#endif
+
+typedef struct abd_stats {
+	kstat_named_t abdstat_struct_size;
+	kstat_named_t abdstat_linear_cnt;
+	kstat_named_t abdstat_linear_data_size;
+	kstat_named_t abdstat_scatter_cnt;
+	kstat_named_t abdstat_scatter_data_size;
+	kstat_named_t abdstat_scatter_chunk_waste;
+	kstat_named_t abdstat_scatter_orders[MAX_ORDER];
+	kstat_named_t abdstat_scatter_page_multi_chunk;
+	kstat_named_t abdstat_scatter_page_multi_zone;
+	kstat_named_t abdstat_scatter_page_alloc_retry;
+	kstat_named_t abdstat_scatter_sg_table_retry;
+} abd_stats_t;
+
+static abd_stats_t abd_stats = {
+	/* Amount of memory occupied by all of the abd_t struct allocations */
+	{ "struct_size",			KSTAT_DATA_UINT64 },
+	/*
+	 * The number of linear ABDs which are currently allocated, excluding
+	 * ABDs which don't own their data (for instance the ones which were
+	 * allocated through abd_get_offset() and abd_get_from_buf()). If an
+	 * ABD takes ownership of its buf then it will become tracked.
+	 */
+	{ "linear_cnt",				KSTAT_DATA_UINT64 },
+	/* Amount of data stored in all linear ABDs tracked by linear_cnt */
+	{ "linear_data_size",			KSTAT_DATA_UINT64 },
+	/*
+	 * The number of scatter ABDs which are currently allocated, excluding
+	 * ABDs which don't own their data (for instance the ones which were
+	 * allocated through abd_get_offset()).
+	 */
+	{ "scatter_cnt",			KSTAT_DATA_UINT64 },
+	/* Amount of data stored in all scatter ABDs tracked by scatter_cnt */
+	{ "scatter_data_size",			KSTAT_DATA_UINT64 },
+	/*
+	 * The amount of space wasted at the end of the last chunk across all
+	 * scatter ABDs tracked by scatter_cnt.
+	 */
+	{ "scatter_chunk_waste",		KSTAT_DATA_UINT64 },
+	/*
+	 * The number of compound allocations of a given order.  These
+	 * allocations are spread over all currently allocated ABDs, and
+	 * act as a measure of memory fragmentation.
+	 */
+	{ { "scatter_order_N",			KSTAT_DATA_UINT64 } },
+	/*
+	 * The number of scatter ABDs which contain multiple chunks.
+	 * ABDs are preferentially allocated from the minimum number of
+	 * contiguous multi-page chunks, a single chunk is optimal.
+	 */
+	{ "scatter_page_multi_chunk",		KSTAT_DATA_UINT64 },
+	/*
+	 * The number of scatter ABDs which are split across memory zones.
+	 * ABDs are preferentially allocated using pages from a single zone.
+	 */
+	{ "scatter_page_multi_zone",		KSTAT_DATA_UINT64 },
+	/*
+	 *  The total number of retries encountered when attempting to
+	 *  allocate the pages to populate the scatter ABD.
+	 */
+	{ "scatter_page_alloc_retry",		KSTAT_DATA_UINT64 },
+	/*
+	 *  The total number of retries encountered when attempting to
+	 *  allocate the sg table for an ABD.
+	 */
+	{ "scatter_sg_table_retry",		KSTAT_DATA_UINT64 },
+};
+
+#define	abd_for_each_sg(abd, sg, n, i)	\
+	for_each_sg(ABD_SCATTER(abd).abd_sgl, sg, n, i)
+
+unsigned zfs_abd_scatter_max_order = MAX_ORDER - 1;
+
+/*
+ * zfs_abd_scatter_min_size is the minimum allocation size to use scatter
+ * ABD's.  Smaller allocations will use linear ABD's which uses
+ * zio_[data_]buf_alloc().
+ *
+ * Scatter ABD's use at least one page each, so sub-page allocations waste
+ * some space when allocated as scatter (e.g. 2KB scatter allocation wastes
+ * half of each page).  Using linear ABD's for small allocations means that
+ * they will be put on slabs which contain many allocations.  This can
+ * improve memory efficiency, but it also makes it much harder for ARC
+ * evictions to actually free pages, because all the buffers on one slab need
+ * to be freed in order for the slab (and underlying pages) to be freed.
+ * Typically, 512B and 1KB kmem caches have 16 buffers per slab, so it's
+ * possible for them to actually waste more memory than scatter (one page per
+ * buf = wasting 3/4 or 7/8th; one buf per slab = wasting 15/16th).
+ *
+ * Spill blocks are typically 512B and are heavily used on systems running
+ * selinux with the default dnode size and the `xattr=sa` property set.
+ *
+ * By default we use linear allocations for 512B and 1KB, and scatter
+ * allocations for larger (1.5KB and up).
+ */
+int zfs_abd_scatter_min_size = 512 * 3;
+
+static kmem_cache_t *abd_cache = NULL;
+static kstat_t *abd_ksp;
+
+static size_t
+abd_chunkcnt_for_bytes(size_t size)
+{
+	return (P2ROUNDUP(size, PAGESIZE) / PAGESIZE);
+}
+
+abd_t *
+abd_alloc_struct(size_t size)
+{
+	/*
+	 * In Linux we do not use the size passed in during ABD
+	 * allocation, so we just ignore it.
+	 */
+	abd_t *abd = kmem_cache_alloc(abd_cache, KM_PUSHPAGE);
+	ASSERT3P(abd, !=, NULL);
+	ABDSTAT_INCR(abdstat_struct_size, sizeof (abd_t));
+
+	return (abd);
+}
+
+void
+abd_free_struct(abd_t *abd)
+{
+	kmem_cache_free(abd_cache, abd);
+	ABDSTAT_INCR(abdstat_struct_size, -(int)sizeof (abd_t));
+}
+
+#ifdef _KERNEL
+/*
+ * Mark zfs data pages so they can be excluded from kernel crash dumps
+ */
+#ifdef _LP64
+#define	ABD_FILE_CACHE_PAGE	0x2F5ABDF11ECAC4E
+
+static inline void
+abd_mark_zfs_page(struct page *page)
+{
+	get_page(page);
+	SetPagePrivate(page);
+	set_page_private(page, ABD_FILE_CACHE_PAGE);
+}
+
+static inline void
+abd_unmark_zfs_page(struct page *page)
+{
+	set_page_private(page, 0UL);
+	ClearPagePrivate(page);
+	put_page(page);
+}
+#else
+#define	abd_mark_zfs_page(page)
+#define	abd_unmark_zfs_page(page)
+#endif /* _LP64 */
+
+#ifndef CONFIG_HIGHMEM
+
+#ifndef __GFP_RECLAIM
+#define	__GFP_RECLAIM		__GFP_WAIT
+#endif
+
+/*
+ * The goal is to minimize fragmentation by preferentially populating ABDs
+ * with higher order compound pages from a single zone.  Allocation size is
+ * progressively decreased until it can be satisfied without performing
+ * reclaim or compaction.  When necessary this function will degenerate to
+ * allocating individual pages and allowing reclaim to satisfy allocations.
+ */
+void
+abd_alloc_chunks(abd_t *abd, size_t size)
+{
+	struct list_head pages;
+	struct sg_table table;
+	struct scatterlist *sg;
+	struct page *page, *tmp_page = NULL;
+	gfp_t gfp = __GFP_NOWARN | GFP_NOIO;
+	gfp_t gfp_comp = (gfp | __GFP_NORETRY | __GFP_COMP) & ~__GFP_RECLAIM;
+	int max_order = MIN(zfs_abd_scatter_max_order, MAX_ORDER - 1);
+	int nr_pages = abd_chunkcnt_for_bytes(size);
+	int chunks = 0, zones = 0;
+	size_t remaining_size;
+	int nid = NUMA_NO_NODE;
+	int alloc_pages = 0;
+
+	INIT_LIST_HEAD(&pages);
+
+	while (alloc_pages < nr_pages) {
+		unsigned chunk_pages;
+		int order;
+
+		order = MIN(highbit64(nr_pages - alloc_pages) - 1, max_order);
+		chunk_pages = (1U << order);
+
+		page = alloc_pages_node(nid, order ? gfp_comp : gfp, order);
+		if (page == NULL) {
+			if (order == 0) {
+				ABDSTAT_BUMP(abdstat_scatter_page_alloc_retry);
+				schedule_timeout_interruptible(1);
+			} else {
+				max_order = MAX(0, order - 1);
+			}
+			continue;
+		}
+
+		list_add_tail(&page->lru, &pages);
+
+		if ((nid != NUMA_NO_NODE) && (page_to_nid(page) != nid))
+			zones++;
+
+		nid = page_to_nid(page);
+		ABDSTAT_BUMP(abdstat_scatter_orders[order]);
+		chunks++;
+		alloc_pages += chunk_pages;
+	}
+
+	ASSERT3S(alloc_pages, ==, nr_pages);
+
+	while (sg_alloc_table(&table, chunks, gfp)) {
+		ABDSTAT_BUMP(abdstat_scatter_sg_table_retry);
+		schedule_timeout_interruptible(1);
+	}
+
+	sg = table.sgl;
+	remaining_size = size;
+	list_for_each_entry_safe(page, tmp_page, &pages, lru) {
+		size_t sg_size = MIN(PAGESIZE << compound_order(page),
+		    remaining_size);
+		sg_set_page(sg, page, sg_size, 0);
+		abd_mark_zfs_page(page);
+		remaining_size -= sg_size;
+
+		sg = sg_next(sg);
+		list_del(&page->lru);
+	}
+
+	/*
+	 * These conditions ensure that a possible transformation to a linear
+	 * ABD would be valid.
+	 */
+	ASSERT(!PageHighMem(sg_page(table.sgl)));
+	ASSERT0(ABD_SCATTER(abd).abd_offset);
+
+	if (table.nents == 1) {
+		/*
+		 * Since there is only one entry, this ABD can be represented
+		 * as a linear buffer.  All single-page (4K) ABD's can be
+		 * represented this way.  Some multi-page ABD's can also be
+		 * represented this way, if we were able to allocate a single
+		 * "chunk" (higher-order "page" which represents a power-of-2
+		 * series of physically-contiguous pages).  This is often the
+		 * case for 2-page (8K) ABD's.
+		 *
+		 * Representing a single-entry scatter ABD as a linear ABD
+		 * has the performance advantage of avoiding the copy (and
+		 * allocation) in abd_borrow_buf_copy / abd_return_buf_copy.
+		 * A performance increase of around 5% has been observed for
+		 * ARC-cached reads (of small blocks which can take advantage
+		 * of this).
+		 *
+		 * Note that this optimization is only possible because the
+		 * pages are always mapped into the kernel's address space.
+		 * This is not the case for highmem pages, so the
+		 * optimization can not be made there.
+		 */
+		abd->abd_flags |= ABD_FLAG_LINEAR;
+		abd->abd_flags |= ABD_FLAG_LINEAR_PAGE;
+		abd->abd_u.abd_linear.abd_sgl = table.sgl;
+		ABD_LINEAR_BUF(abd) = page_address(sg_page(table.sgl));
+	} else if (table.nents > 1) {
+		ABDSTAT_BUMP(abdstat_scatter_page_multi_chunk);
+		abd->abd_flags |= ABD_FLAG_MULTI_CHUNK;
+
+		if (zones) {
+			ABDSTAT_BUMP(abdstat_scatter_page_multi_zone);
+			abd->abd_flags |= ABD_FLAG_MULTI_ZONE;
+		}
+
+		ABD_SCATTER(abd).abd_sgl = table.sgl;
+		ABD_SCATTER(abd).abd_nents = table.nents;
+	}
+}
+#else
+
+/*
+ * Allocate N individual pages to construct a scatter ABD.  This function
+ * makes no attempt to request contiguous pages and requires the minimal
+ * number of kernel interfaces.  It's designed for maximum compatibility.
+ */
+void
+abd_alloc_chunks(abd_t *abd, size_t size)
+{
+	struct scatterlist *sg = NULL;
+	struct sg_table table;
+	struct page *page;
+	gfp_t gfp = __GFP_NOWARN | GFP_NOIO;
+	int nr_pages = abd_chunkcnt_for_bytes(size);
+	int i = 0;
+
+	while (sg_alloc_table(&table, nr_pages, gfp)) {
+		ABDSTAT_BUMP(abdstat_scatter_sg_table_retry);
+		schedule_timeout_interruptible(1);
+	}
+
+	ASSERT3U(table.nents, ==, nr_pages);
+	ABD_SCATTER(abd).abd_sgl = table.sgl;
+	ABD_SCATTER(abd).abd_nents = nr_pages;
+
+	abd_for_each_sg(abd, sg, nr_pages, i) {
+		while ((page = __page_cache_alloc(gfp)) == NULL) {
+			ABDSTAT_BUMP(abdstat_scatter_page_alloc_retry);
+			schedule_timeout_interruptible(1);
+		}
+
+		ABDSTAT_BUMP(abdstat_scatter_orders[0]);
+		sg_set_page(sg, page, PAGESIZE, 0);
+		abd_mark_zfs_page(page);
+	}
+
+	if (nr_pages > 1) {
+		ABDSTAT_BUMP(abdstat_scatter_page_multi_chunk);
+		abd->abd_flags |= ABD_FLAG_MULTI_CHUNK;
+	}
+}
+#endif /* !CONFIG_HIGHMEM */
+
+/*
+ * This must be called if any of the sg_table allocation functions
+ * are called.
+ */
+static void
+abd_free_sg_table(abd_t *abd)
+{
+	struct sg_table table;
+
+	table.sgl = ABD_SCATTER(abd).abd_sgl;
+	table.nents = table.orig_nents = ABD_SCATTER(abd).abd_nents;
+	sg_free_table(&table);
+}
+
+void
+abd_free_chunks(abd_t *abd)
+{
+	struct scatterlist *sg = NULL;
+	struct page *page;
+	int nr_pages = ABD_SCATTER(abd).abd_nents;
+	int order, i = 0;
+
+	if (abd->abd_flags & ABD_FLAG_MULTI_ZONE)
+		ABDSTAT_BUMPDOWN(abdstat_scatter_page_multi_zone);
+
+	if (abd->abd_flags & ABD_FLAG_MULTI_CHUNK)
+		ABDSTAT_BUMPDOWN(abdstat_scatter_page_multi_chunk);
+
+	abd_for_each_sg(abd, sg, nr_pages, i) {
+		page = sg_page(sg);
+		abd_unmark_zfs_page(page);
+		order = compound_order(page);
+		__free_pages(page, order);
+		ASSERT3U(sg->length, <=, PAGE_SIZE << order);
+		ABDSTAT_BUMPDOWN(abdstat_scatter_orders[order]);
+	}
+	abd_free_sg_table(abd);
+}
+
+#else /* _KERNEL */
+
+#ifndef PAGE_SHIFT
+#define	PAGE_SHIFT (highbit64(PAGESIZE)-1)
+#endif
+
+struct page;
+
+#define	zfs_kmap_atomic(chunk, km)	((void *)chunk)
+#define	zfs_kunmap_atomic(addr, km)	do { (void)(addr); } while (0)
+#define	local_irq_save(flags)		do { (void)(flags); } while (0)
+#define	local_irq_restore(flags)	do { (void)(flags); } while (0)
+#define	nth_page(pg, i) \
+	((struct page *)((void *)(pg) + (i) * PAGESIZE))
+
+struct scatterlist {
+	struct page *page;
+	int length;
+	int end;
+};
+
+static void
+sg_init_table(struct scatterlist *sg, int nr)
+{
+	memset(sg, 0, nr * sizeof (struct scatterlist));
+	sg[nr - 1].end = 1;
+}
+
+/*
+ * This must be called if any of the sg_table allocation functions
+ * are called.
+ */
+static void
+abd_free_sg_table(abd_t *abd)
+{
+	int nents = ABD_SCATTER(abd).abd_nents;
+	vmem_free(ABD_SCATTER(abd).abd_sgl,
+	    nents * sizeof (struct scatterlist));
+}
+
+#define	for_each_sg(sgl, sg, nr, i)	\
+	for ((i) = 0, (sg) = (sgl); (i) < (nr); (i)++, (sg) = sg_next(sg))
+
+static inline void
+sg_set_page(struct scatterlist *sg, struct page *page, unsigned int len,
+    unsigned int offset)
+{
+	/* currently we don't use offset */
+	ASSERT(offset == 0);
+	sg->page = page;
+	sg->length = len;
+}
+
+static inline struct page *
+sg_page(struct scatterlist *sg)
+{
+	return (sg->page);
+}
+
+static inline struct scatterlist *
+sg_next(struct scatterlist *sg)
+{
+	if (sg->end)
+		return (NULL);
+
+	return (sg + 1);
+}
+
+void
+abd_alloc_chunks(abd_t *abd, size_t size)
+{
+	unsigned nr_pages = abd_chunkcnt_for_bytes(size);
+	struct scatterlist *sg;
+	int i;
+
+	ABD_SCATTER(abd).abd_sgl = vmem_alloc(nr_pages *
+	    sizeof (struct scatterlist), KM_SLEEP);
+	sg_init_table(ABD_SCATTER(abd).abd_sgl, nr_pages);
+
+	abd_for_each_sg(abd, sg, nr_pages, i) {
+		struct page *p = umem_alloc_aligned(PAGESIZE, 64, KM_SLEEP);
+		sg_set_page(sg, p, PAGESIZE, 0);
+	}
+	ABD_SCATTER(abd).abd_nents = nr_pages;
+}
+
+void
+abd_free_chunks(abd_t *abd)
+{
+	int i, n = ABD_SCATTER(abd).abd_nents;
+	struct scatterlist *sg;
+
+	abd_for_each_sg(abd, sg, n, i) {
+		for (int j = 0; j < sg->length; j += PAGESIZE) {
+			struct page *p = nth_page(sg_page(sg), j >> PAGE_SHIFT);
+			umem_free(p, PAGESIZE);
+		}
+	}
+	abd_free_sg_table(abd);
+}
+
+#endif /* _KERNEL */
+
+boolean_t
+abd_size_alloc_linear(size_t size)
+{
+	return (size < zfs_abd_scatter_min_size ? B_TRUE : B_FALSE);
+}
+
+void
+abd_update_scatter_stats(abd_t *abd, abd_stats_op_t op)
+{
+	ASSERT(op == ABDSTAT_INCR || op == ABDSTAT_DECR);
+	if (op == ABDSTAT_INCR) {
+		ABDSTAT_BUMP(abdstat_scatter_cnt);
+		ABDSTAT_INCR(abdstat_scatter_data_size, abd->abd_size);
+		ABDSTAT_INCR(abdstat_scatter_chunk_waste,
+		    P2ROUNDUP(abd->abd_size, PAGESIZE) - abd->abd_size);
+	} else {
+		ABDSTAT_BUMPDOWN(abdstat_scatter_cnt);
+		ABDSTAT_INCR(abdstat_scatter_data_size, -(int)abd->abd_size);
+		ABDSTAT_INCR(abdstat_scatter_chunk_waste,
+		    (int)abd->abd_size
+		    -(int)P2ROUNDUP(abd->abd_size, PAGESIZE));
+	}
+}
+
+void
+abd_update_linear_stats(abd_t *abd, abd_stats_op_t op)
+{
+	ASSERT(op == ABDSTAT_INCR || op == ABDSTAT_DECR);
+	if (op == ABDSTAT_INCR) {
+		ABDSTAT_BUMP(abdstat_linear_cnt);
+		ABDSTAT_INCR(abdstat_linear_data_size, abd->abd_size);
+	} else {
+		ABDSTAT_BUMPDOWN(abdstat_linear_cnt);
+		ABDSTAT_INCR(abdstat_linear_data_size, -(int)abd->abd_size);
+	}
+}
+
+void
+abd_verify_scatter(abd_t *abd)
+{
+	size_t n;
+	int i = 0;
+	struct scatterlist *sg = NULL;
+
+	ASSERT3U(ABD_SCATTER(abd).abd_nents, >, 0);
+	ASSERT3U(ABD_SCATTER(abd).abd_offset, <,
+	    ABD_SCATTER(abd).abd_sgl->length);
+	n = ABD_SCATTER(abd).abd_nents;
+	abd_for_each_sg(abd, sg, n, i) {
+		ASSERT3P(sg_page(sg), !=, NULL);
+	}
+}
+
+void
+abd_init(void)
+{
+	int i;
+
+	abd_cache = kmem_cache_create("abd_t", sizeof (abd_t),
+	    0, NULL, NULL, NULL, NULL, NULL, 0);
+
+	abd_ksp = kstat_create("zfs", 0, "abdstats", "misc", KSTAT_TYPE_NAMED,
+	    sizeof (abd_stats) / sizeof (kstat_named_t), KSTAT_FLAG_VIRTUAL);
+	if (abd_ksp != NULL) {
+		for (i = 0; i < MAX_ORDER; i++) {
+			snprintf(abd_stats.abdstat_scatter_orders[i].name,
+			    KSTAT_STRLEN, "scatter_order_%d", i);
+			abd_stats.abdstat_scatter_orders[i].data_type =
+			    KSTAT_DATA_UINT64;
+		}
+		abd_ksp->ks_data = &abd_stats;
+		kstat_install(abd_ksp);
+	}
+}
+
+void
+abd_fini(void)
+{
+	if (abd_ksp != NULL) {
+		kstat_delete(abd_ksp);
+		abd_ksp = NULL;
+	}
+
+	if (abd_cache) {
+		kmem_cache_destroy(abd_cache);
+		abd_cache = NULL;
+	}
+}
+
+void
+abd_free_linear_page(abd_t *abd)
+{
+	/* Transform it back into a scatter ABD for freeing */
+	struct scatterlist *sg = abd->abd_u.abd_linear.abd_sgl;
+	abd->abd_flags &= ~ABD_FLAG_LINEAR;
+	abd->abd_flags &= ~ABD_FLAG_LINEAR_PAGE;
+	ABD_SCATTER(abd).abd_nents = 1;
+	ABD_SCATTER(abd).abd_offset = 0;
+	ABD_SCATTER(abd).abd_sgl = sg;
+	abd_free_chunks(abd);
+
+	zfs_refcount_destroy(&abd->abd_children);
+	abd_update_scatter_stats(abd, ABDSTAT_DECR);
+	abd_free_struct(abd);
+}
+
+/*
+ * If we're going to use this ABD for doing I/O using the block layer, the
+ * consumer of the ABD data doesn't care if it's scattered or not, and we don't
+ * plan to store this ABD in memory for a long period of time, we should
+ * allocate the ABD type that requires the least data copying to do the I/O.
+ *
+ * On Linux the optimal thing to do would be to use abd_get_offset() and
+ * construct a new ABD which shares the original pages thereby eliminating
+ * the copy.  But for the moment a new linear ABD is allocated until this
+ * performance optimization can be implemented.
+ */
+abd_t *
+abd_alloc_for_io(size_t size, boolean_t is_metadata)
+{
+	return (abd_alloc(size, is_metadata));
+}
+
+abd_t *
+abd_get_offset_scatter(abd_t *sabd, size_t off)
+{
+	abd_t *abd = NULL;
+	int i = 0;
+	struct scatterlist *sg = NULL;
+
+	abd_verify(sabd);
+	ASSERT3U(off, <=, sabd->abd_size);
+
+	size_t new_offset = ABD_SCATTER(sabd).abd_offset + off;
+
+	abd = abd_alloc_struct(0);
+
+	/*
+	 * Even if this buf is filesystem metadata, we only track that
+	 * if we own the underlying data buffer, which is not true in
+	 * this case. Therefore, we don't ever use ABD_FLAG_META here.
+	 */
+	abd->abd_flags = 0;
+
+	abd_for_each_sg(sabd, sg, ABD_SCATTER(sabd).abd_nents, i) {
+		if (new_offset < sg->length)
+			break;
+		new_offset -= sg->length;
+	}
+
+	ABD_SCATTER(abd).abd_sgl = sg;
+	ABD_SCATTER(abd).abd_offset = new_offset;
+	ABD_SCATTER(abd).abd_nents = ABD_SCATTER(sabd).abd_nents - i;
+
+	return (abd);
+}
+
+/*
+ * Initialize the abd_iter.
+ */
+void
+abd_iter_init(struct abd_iter *aiter, abd_t *abd)
+{
+	abd_verify(abd);
+	aiter->iter_abd = abd;
+	aiter->iter_mapaddr = NULL;
+	aiter->iter_mapsize = 0;
+	aiter->iter_pos = 0;
+	if (abd_is_linear(abd)) {
+		aiter->iter_offset = 0;
+		aiter->iter_sg = NULL;
+	} else {
+		aiter->iter_offset = ABD_SCATTER(abd).abd_offset;
+		aiter->iter_sg = ABD_SCATTER(abd).abd_sgl;
+	}
+}
+
+/*
+ * This is just a helper function to see if we have exhausted the
+ * abd_iter and reached the end.
+ */
+boolean_t
+abd_iter_at_end(struct abd_iter *aiter)
+{
+	return (aiter->iter_pos == aiter->iter_abd->abd_size);
+}
+
+/*
+ * Advance the iterator by a certain amount. Cannot be called when a chunk is
+ * in use. This can be safely called when the aiter has already exhausted, in
+ * which case this does nothing.
+ */
+void
+abd_iter_advance(struct abd_iter *aiter, size_t amount)
+{
+	ASSERT3P(aiter->iter_mapaddr, ==, NULL);
+	ASSERT0(aiter->iter_mapsize);
+
+	/* There's nothing left to advance to, so do nothing */
+	if (abd_iter_at_end(aiter))
+		return;
+
+	aiter->iter_pos += amount;
+	aiter->iter_offset += amount;
+	if (!abd_is_linear(aiter->iter_abd)) {
+		while (aiter->iter_offset >= aiter->iter_sg->length) {
+			aiter->iter_offset -= aiter->iter_sg->length;
+			aiter->iter_sg = sg_next(aiter->iter_sg);
+			if (aiter->iter_sg == NULL) {
+				ASSERT0(aiter->iter_offset);
+				break;
+			}
+		}
+	}
+}
+
+/*
+ * Map the current chunk into aiter. This can be safely called when the aiter
+ * has already exhausted, in which case this does nothing.
+ */
+void
+abd_iter_map(struct abd_iter *aiter)
+{
+	void *paddr;
+	size_t offset = 0;
+
+	ASSERT3P(aiter->iter_mapaddr, ==, NULL);
+	ASSERT0(aiter->iter_mapsize);
+
+	/* There's nothing left to iterate over, so do nothing */
+	if (abd_iter_at_end(aiter))
+		return;
+
+	if (abd_is_linear(aiter->iter_abd)) {
+		ASSERT3U(aiter->iter_pos, ==, aiter->iter_offset);
+		offset = aiter->iter_offset;
+		aiter->iter_mapsize = aiter->iter_abd->abd_size - offset;
+		paddr = ABD_LINEAR_BUF(aiter->iter_abd);
+	} else {
+		offset = aiter->iter_offset;
+		aiter->iter_mapsize = MIN(aiter->iter_sg->length - offset,
+		    aiter->iter_abd->abd_size - aiter->iter_pos);
+
+		paddr = zfs_kmap_atomic(sg_page(aiter->iter_sg),
+		    km_table[aiter->iter_km]);
+	}
+
+	aiter->iter_mapaddr = (char *)paddr + offset;
+}
+
+/*
+ * Unmap the current chunk from aiter. This can be safely called when the aiter
+ * has already exhausted, in which case this does nothing.
+ */
+void
+abd_iter_unmap(struct abd_iter *aiter)
+{
+	/* There's nothing left to unmap, so do nothing */
+	if (abd_iter_at_end(aiter))
+		return;
+
+	if (!abd_is_linear(aiter->iter_abd)) {
+		/* LINTED E_FUNC_SET_NOT_USED */
+		zfs_kunmap_atomic(aiter->iter_mapaddr - aiter->iter_offset,
+		    km_table[aiter->iter_km]);
+	}
+
+	ASSERT3P(aiter->iter_mapaddr, !=, NULL);
+	ASSERT3U(aiter->iter_mapsize, >, 0);
+
+	aiter->iter_mapaddr = NULL;
+	aiter->iter_mapsize = 0;
+}
+
+void
+abd_enter_critical(unsigned long flags)
+{
+	local_irq_save(flags);
+}
+
+void
+abd_exit_critical(unsigned long flags)
+{
+	local_irq_restore(flags);
+}
+
+#if defined(_KERNEL)
+/*
+ * bio_nr_pages for ABD.
+ * @off is the offset in @abd
+ */
+unsigned long
+abd_nr_pages_off(abd_t *abd, unsigned int size, size_t off)
+{
+	unsigned long pos;
+
+	if (abd_is_linear(abd))
+		pos = (unsigned long)abd_to_buf(abd) + off;
+	else
+		pos = ABD_SCATTER(abd).abd_offset + off;
+
+	return ((pos + size + PAGESIZE - 1) >> PAGE_SHIFT) -
+	    (pos >> PAGE_SHIFT);
+}
+
+/*
+ * bio_map for scatter ABD.
+ * @off is the offset in @abd
+ * Remaining IO size is returned
+ */
+unsigned int
+abd_scatter_bio_map_off(struct bio *bio, abd_t *abd,
+    unsigned int io_size, size_t off)
+{
+	int i;
+	struct abd_iter aiter;
+
+	ASSERT(!abd_is_linear(abd));
+	ASSERT3U(io_size, <=, abd->abd_size - off);
+
+	abd_iter_init(&aiter, abd);
+	abd_iter_advance(&aiter, off);
+
+	for (i = 0; i < bio->bi_max_vecs; i++) {
+		struct page *pg;
+		size_t len, sgoff, pgoff;
+		struct scatterlist *sg;
+
+		if (io_size <= 0)
+			break;
+
+		sg = aiter.iter_sg;
+		sgoff = aiter.iter_offset;
+		pgoff = sgoff & (PAGESIZE - 1);
+		len = MIN(io_size, PAGESIZE - pgoff);
+		ASSERT(len > 0);
+
+		pg = nth_page(sg_page(sg), sgoff >> PAGE_SHIFT);
+		if (bio_add_page(bio, pg, len, pgoff) != len)
+			break;
+
+		io_size -= len;
+		abd_iter_advance(&aiter, len);
+	}
+
+	return (io_size);
+}
+
+/* Tunable Parameters */
+module_param(zfs_abd_scatter_enabled, int, 0644);
+MODULE_PARM_DESC(zfs_abd_scatter_enabled,
+	"Toggle whether ABD allocations must be linear.");
+module_param(zfs_abd_scatter_min_size, int, 0644);
+MODULE_PARM_DESC(zfs_abd_scatter_min_size,
+	"Minimum size of scatter allocations.");
+/* CSTYLED */
+module_param(zfs_abd_scatter_max_order, uint, 0644);
+MODULE_PARM_DESC(zfs_abd_scatter_max_order,
+	"Maximum order allocation used for a scatter ABD.");
+#endif
diff --git a/module/zfs/Makefile.in b/module/zfs/Makefile.in
index 6737336ca..3a9663997 100644
--- a/module/zfs/Makefile.in
+++ b/module/zfs/Makefile.in
@@ -14,6 +14,7 @@ ccflags-y += $(NO_UNUSED_BUT_SET_VARIABLE)
 # Suppress unused-value warnings in sparc64 architecture headers
 ccflags-$(CONFIG_SPARC64) += -Wno-unused-value
 
+$(MODULE)-objs += abd.o
 $(MODULE)-objs += aggsum.o
 $(MODULE)-objs += arc.o
 $(MODULE)-objs += blkptr.o
diff --git a/module/os/freebsd/zfs/abd.c b/module/zfs/abd.c
index 888a113a4..2e4554da7 100644
--- a/module/os/freebsd/zfs/abd.c
+++ b/module/zfs/abd.c
@@ -1,17 +1,26 @@
 /*
- * This file and its contents are supplied under the terms of the
- * Common Development and Distribution License ("CDDL"), version 1.0.
- * You may only use this file in accordance with the terms of version
- * 1.0 of the CDDL.
+ * CDDL HEADER START
  *
- * A full copy of the text of the CDDL should have accompanied this
- * source.  A copy of the CDDL is also available via the Internet at
- * http://www.illumos.org/license/CDDL.
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://www.opensolaris.org/os/licensing.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
  */
-
 /*
  * Copyright (c) 2014 by Chunwei Chen. All rights reserved.
- * Copyright (c) 2016 by Delphix. All rights reserved.
+ * Copyright (c) 2019 by Delphix. All rights reserved.
  */
 
 /*
@@ -50,11 +59,6 @@
  *                                      +----------------->| chunk N-1 |
  *                                                         +-----------+
  *
- * Using a large proportion of scattered ABDs decreases ARC fragmentation since
- * when we are at the limit of allocatable space, using equal-size chunks will
- * allow us to quickly reclaim enough space for a new large allocation (assuming
- * it is also scattered).
- *
  * In addition to directly allocating a linear or scattered ABD, it is also
  * possible to create an ABD by requesting the "sub-ABD" starting at an offset
  * within an existing ABD. In linear buffers this is simple (set abd_buf of
@@ -83,186 +87,55 @@
  * compare, copy, read, write, and fill with zeroes. If you need a custom
  * function which progressively accesses the whole ABD, use the abd_iterate_*
  * functions.
+ *
+ * It is possible to make all ABDs linear by setting zfs_abd_scatter_enabled to
+ * B_FALSE.
  */
 
-#include <sys/abd.h>
+#include <sys/abd_impl.h>
 #include <sys/param.h>
 #include <sys/zio.h>
 #include <sys/zfs_context.h>
 #include <sys/zfs_znode.h>
 
-typedef struct abd_stats {
-	kstat_named_t abdstat_struct_size;
-	kstat_named_t abdstat_scatter_cnt;
-	kstat_named_t abdstat_scatter_data_size;
-	kstat_named_t abdstat_scatter_chunk_waste;
-	kstat_named_t abdstat_linear_cnt;
-	kstat_named_t abdstat_linear_data_size;
-} abd_stats_t;
-
-static abd_stats_t abd_stats = {
-	/* Amount of memory occupied by all of the abd_t struct allocations */
-	{ "struct_size",			KSTAT_DATA_UINT64 },
-	/*
-	 * The number of scatter ABDs which are currently allocated, excluding
-	 * ABDs which don't own their data (for instance the ones which were
-	 * allocated through abd_get_offset()).
-	 */
-	{ "scatter_cnt",			KSTAT_DATA_UINT64 },
-	/* Amount of data stored in all scatter ABDs tracked by scatter_cnt */
-	{ "scatter_data_size",			KSTAT_DATA_UINT64 },
-	/*
-	 * The amount of space wasted at the end of the last chunk across all
-	 * scatter ABDs tracked by scatter_cnt.
-	 */
-	{ "scatter_chunk_waste",		KSTAT_DATA_UINT64 },
-	/*
-	 * The number of linear ABDs which are currently allocated, excluding
-	 * ABDs which don't own their data (for instance the ones which were
-	 * allocated through abd_get_offset() and abd_get_from_buf()). If an
-	 * ABD takes ownership of its buf then it will become tracked.
-	 */
-	{ "linear_cnt",				KSTAT_DATA_UINT64 },
-	/* Amount of data stored in all linear ABDs tracked by linear_cnt */
-	{ "linear_data_size",			KSTAT_DATA_UINT64 },
-};
-
-#define	ABDSTAT(stat)		(abd_stats.stat.value.ui64)
-#define	ABDSTAT_INCR(stat, val) \
-	atomic_add_64(&abd_stats.stat.value.ui64, (val))
-#define	ABDSTAT_BUMP(stat)	ABDSTAT_INCR(stat, 1)
-#define	ABDSTAT_BUMPDOWN(stat)	ABDSTAT_INCR(stat, -1)
-
-/*
- * It is possible to make all future ABDs be linear by setting this to B_FALSE.
- * Otherwise, ABDs are allocated scattered by default unless the caller uses
- * abd_alloc_linear().
- */
-boolean_t zfs_abd_scatter_enabled = B_TRUE;
-
-/*
- * The size of the chunks ABD allocates. Because the sizes allocated from the
- * kmem_cache can't change, this tunable can only be modified at boot. Changing
- * it at runtime would cause ABD iteration to work incorrectly for ABDs which
- * were allocated with the old size, so a safeguard has been put in place which
- * will cause the machine to panic if you change it and try to access the data
- * within a scattered ABD.
- */
-size_t zfs_abd_chunk_size = 4096;
-
-#if defined(_KERNEL)
-SYSCTL_DECL(_vfs_zfs);
-
-SYSCTL_INT(_vfs_zfs, OID_AUTO, abd_scatter_enabled, CTLFLAG_RWTUN,
-	&zfs_abd_scatter_enabled, 0, "Enable scattered ARC data buffers");
-SYSCTL_ULONG(_vfs_zfs, OID_AUTO, abd_chunk_size, CTLFLAG_RDTUN,
-	&zfs_abd_chunk_size, 0, "The size of the chunks ABD allocates");
-#endif
-
-kmem_cache_t *abd_chunk_cache;
-static kstat_t *abd_ksp;
-
-extern inline boolean_t abd_is_linear(abd_t *abd);
-extern inline void abd_copy(abd_t *dabd, abd_t *sabd, size_t size);
-extern inline void abd_copy_from_buf(abd_t *abd, const void *buf, size_t size);
-extern inline void abd_copy_to_buf(void* buf, abd_t *abd, size_t size);
-extern inline int abd_cmp_buf(abd_t *abd, const void *buf, size_t size);
-extern inline void abd_zero(abd_t *abd, size_t size);
+/* see block comment above for description */
+int zfs_abd_scatter_enabled = B_TRUE;
 
-static void *
-abd_alloc_chunk()
+boolean_t
+abd_is_linear(abd_t *abd)
 {
-	void *c = kmem_cache_alloc(abd_chunk_cache, KM_PUSHPAGE);
-	ASSERT3P(c, !=, NULL);
-	return (c);
+	return ((abd->abd_flags & ABD_FLAG_LINEAR) != 0 ? B_TRUE : B_FALSE);
 }
 
-static void
-abd_free_chunk(void *c)
+boolean_t
+abd_is_linear_page(abd_t *abd)
 {
-	kmem_cache_free(abd_chunk_cache, c);
+	return ((abd->abd_flags & ABD_FLAG_LINEAR_PAGE) != 0 ?
+	    B_TRUE : B_FALSE);
 }
 
 void
-abd_init(void)
-{
-	abd_chunk_cache = kmem_cache_create("abd_chunk", zfs_abd_chunk_size, 0,
-	    NULL, NULL, NULL, NULL, 0, KMC_NOTOUCH | KMC_NODEBUG);
-
-	abd_ksp = kstat_create("zfs", 0, "abdstats", "misc", KSTAT_TYPE_NAMED,
-	    sizeof (abd_stats) / sizeof (kstat_named_t), KSTAT_FLAG_VIRTUAL);
-	if (abd_ksp != NULL) {
-		abd_ksp->ks_data = &abd_stats;
-		kstat_install(abd_ksp);
-	}
-}
-
-void
-abd_fini(void)
-{
-	if (abd_ksp != NULL) {
-		kstat_delete(abd_ksp);
-		abd_ksp = NULL;
-	}
-
-	kmem_cache_destroy(abd_chunk_cache);
-	abd_chunk_cache = NULL;
-}
-
-static inline size_t
-abd_chunkcnt_for_bytes(size_t size)
-{
-	return (P2ROUNDUP(size, zfs_abd_chunk_size) / zfs_abd_chunk_size);
-}
-
-static inline size_t
-abd_scatter_chunkcnt(abd_t *abd)
-{
-	ASSERT(!abd_is_linear(abd));
-	return (abd_chunkcnt_for_bytes(
-	    abd->abd_u.abd_scatter.abd_offset + abd->abd_size));
-}
-
-static inline void
 abd_verify(abd_t *abd)
 {
 	ASSERT3U(abd->abd_size, >, 0);
 	ASSERT3U(abd->abd_size, <=, SPA_MAXBLOCKSIZE);
 	ASSERT3U(abd->abd_flags, ==, abd->abd_flags & (ABD_FLAG_LINEAR |
-	    ABD_FLAG_OWNER | ABD_FLAG_META));
+	    ABD_FLAG_OWNER | ABD_FLAG_META | ABD_FLAG_MULTI_ZONE |
+	    ABD_FLAG_MULTI_CHUNK | ABD_FLAG_LINEAR_PAGE));
 	IMPLY(abd->abd_parent != NULL, !(abd->abd_flags & ABD_FLAG_OWNER));
 	IMPLY(abd->abd_flags & ABD_FLAG_META, abd->abd_flags & ABD_FLAG_OWNER);
 	if (abd_is_linear(abd)) {
-		ASSERT3P(abd->abd_u.abd_linear.abd_buf, !=, NULL);
+		ASSERT3P(ABD_LINEAR_BUF(abd), !=, NULL);
 	} else {
-		ASSERT3U(abd->abd_u.abd_scatter.abd_offset, <,
-		    zfs_abd_chunk_size);
-		size_t n = abd_scatter_chunkcnt(abd);
-		for (int i = 0; i < n; i++) {
-			ASSERT3P(
-			    abd->abd_u.abd_scatter.abd_chunks[i], !=, NULL);
-		}
+		abd_verify_scatter(abd);
 	}
 }
 
-static inline abd_t *
-abd_alloc_struct(size_t chunkcnt)
-{
-	size_t size = offsetof(abd_t, abd_u.abd_scatter.abd_chunks[chunkcnt]);
-	abd_t *abd = kmem_alloc(size, KM_PUSHPAGE);
-	ASSERT3P(abd, !=, NULL);
-	ABDSTAT_INCR(abdstat_struct_size, size);
-
-	return (abd);
-}
-
-static inline void
-abd_free_struct(abd_t *abd)
+uint_t
+abd_get_size(abd_t *abd)
 {
-	size_t chunkcnt = abd_is_linear(abd) ? 0 : abd_scatter_chunkcnt(abd);
-	int size = offsetof(abd_t, abd_u.abd_scatter.abd_chunks[chunkcnt]);
-	kmem_free(abd, size);
-	ABDSTAT_INCR(abdstat_struct_size, -size);
+	abd_verify(abd);
+	return (abd->abd_size);
 }
 
 /*
@@ -272,15 +145,16 @@ abd_free_struct(abd_t *abd)
 abd_t *
 abd_alloc(size_t size, boolean_t is_metadata)
 {
-	if (!zfs_abd_scatter_enabled || size <= zfs_abd_chunk_size)
+	if (!zfs_abd_scatter_enabled || abd_size_alloc_linear(size))
 		return (abd_alloc_linear(size, is_metadata));
 
 	VERIFY3U(size, <=, SPA_MAXBLOCKSIZE);
 
-	size_t n = abd_chunkcnt_for_bytes(size);
-	abd_t *abd = abd_alloc_struct(n);
-
+	abd_t *abd = abd_alloc_struct(size);
 	abd->abd_flags = ABD_FLAG_OWNER;
+	abd->abd_u.abd_scatter.abd_offset = 0;
+	abd_alloc_chunks(abd, size);
+
 	if (is_metadata) {
 		abd->abd_flags |= ABD_FLAG_META;
 	}
@@ -288,19 +162,7 @@ abd_alloc(size_t size, boolean_t is_metadata)
 	abd->abd_parent = NULL;
 	zfs_refcount_create(&abd->abd_children);
 
-	abd->abd_u.abd_scatter.abd_offset = 0;
-	abd->abd_u.abd_scatter.abd_chunk_size = zfs_abd_chunk_size;
-
-	for (int i = 0; i < n; i++) {
-		void *c = abd_alloc_chunk();
-		ASSERT3P(c, !=, NULL);
-		abd->abd_u.abd_scatter.abd_chunks[i] = c;
-	}
-
-	ABDSTAT_BUMP(abdstat_scatter_cnt);
-	ABDSTAT_INCR(abdstat_scatter_data_size, size);
-	ABDSTAT_INCR(abdstat_scatter_chunk_waste,
-	    n * zfs_abd_chunk_size - size);
+	abd_update_scatter_stats(abd, ABDSTAT_INCR);
 
 	return (abd);
 }
@@ -308,17 +170,32 @@ abd_alloc(size_t size, boolean_t is_metadata)
 static void
 abd_free_scatter(abd_t *abd)
 {
-	size_t n = abd_scatter_chunkcnt(abd);
-	for (int i = 0; i < n; i++) {
-		abd_free_chunk(abd->abd_u.abd_scatter.abd_chunks[i]);
-	}
+	abd_free_chunks(abd);
 
 	zfs_refcount_destroy(&abd->abd_children);
-	ABDSTAT_BUMPDOWN(abdstat_scatter_cnt);
-	ABDSTAT_INCR(abdstat_scatter_data_size, -(int)abd->abd_size);
-	ABDSTAT_INCR(abdstat_scatter_chunk_waste,
-	    abd->abd_size - n * zfs_abd_chunk_size);
+	abd_update_scatter_stats(abd, ABDSTAT_DECR);
+	abd_free_struct(abd);
+}
 
+/*
+ * Free an ABD allocated from abd_get_offset() or abd_get_from_buf(). Will not
+ * free the underlying scatterlist or buffer.
+ */
+void
+abd_put(abd_t *abd)
+{
+	if (abd == NULL)
+		return;
+
+	abd_verify(abd);
+	ASSERT(!(abd->abd_flags & ABD_FLAG_OWNER));
+
+	if (abd->abd_parent != NULL) {
+		(void) zfs_refcount_remove_many(&abd->abd_parent->abd_children,
+		    abd->abd_size, abd);
+	}
+
+	zfs_refcount_destroy(&abd->abd_children);
 	abd_free_struct(abd);
 }
 
@@ -343,13 +220,12 @@ abd_alloc_linear(size_t size, boolean_t is_metadata)
 	zfs_refcount_create(&abd->abd_children);
 
 	if (is_metadata) {
-		abd->abd_u.abd_linear.abd_buf = zio_buf_alloc(size);
+		ABD_LINEAR_BUF(abd) = zio_buf_alloc(size);
 	} else {
-		abd->abd_u.abd_linear.abd_buf = zio_data_buf_alloc(size);
+		ABD_LINEAR_BUF(abd) = zio_data_buf_alloc(size);
 	}
 
-	ABDSTAT_BUMP(abdstat_linear_cnt);
-	ABDSTAT_INCR(abdstat_linear_data_size, size);
+	abd_update_linear_stats(abd, ABDSTAT_INCR);
 
 	return (abd);
 }
@@ -357,15 +233,18 @@ abd_alloc_linear(size_t size, boolean_t is_metadata)
 static void
 abd_free_linear(abd_t *abd)
 {
+	if (abd_is_linear_page(abd)) {
+		abd_free_linear_page(abd);
+		return;
+	}
 	if (abd->abd_flags & ABD_FLAG_META) {
-		zio_buf_free(abd->abd_u.abd_linear.abd_buf, abd->abd_size);
+		zio_buf_free(ABD_LINEAR_BUF(abd), abd->abd_size);
 	} else {
-		zio_data_buf_free(abd->abd_u.abd_linear.abd_buf, abd->abd_size);
+		zio_data_buf_free(ABD_LINEAR_BUF(abd), abd->abd_size);
 	}
 
 	zfs_refcount_destroy(&abd->abd_children);
-	ABDSTAT_BUMPDOWN(abdstat_linear_cnt);
-	ABDSTAT_INCR(abdstat_linear_data_size, -(int)abd->abd_size);
+	abd_update_linear_stats(abd, ABDSTAT_DECR);
 
 	abd_free_struct(abd);
 }
@@ -397,7 +276,8 @@ abd_t *
 abd_alloc_sametype(abd_t *sabd, size_t size)
 {
 	boolean_t is_metadata = (sabd->abd_flags & ABD_FLAG_META) != 0;
-	if (abd_is_linear(sabd)) {
+	if (abd_is_linear(sabd) &&
+	    !abd_is_linear_page(sabd)) {
 		return (abd_alloc_linear(size, is_metadata));
 	} else {
 		return (abd_alloc(size, is_metadata));
@@ -405,31 +285,14 @@ abd_alloc_sametype(abd_t *sabd, size_t size)
 }
 
 /*
- * If we're going to use this ABD for doing I/O using the block layer, the
- * consumer of the ABD data doesn't care if it's scattered or not, and we don't
- * plan to store this ABD in memory for a long period of time, we should
- * allocate the ABD type that requires the least data copying to do the I/O.
- *
- * Currently this is linear ABDs, however if ldi_strategy() can ever issue I/Os
- * using a scatter/gather list we should switch to that and replace this call
- * with vanilla abd_alloc().
- */
-abd_t *
-abd_alloc_for_io(size_t size, boolean_t is_metadata)
-{
-	return (abd_alloc_linear(size, is_metadata));
-}
-
-/*
  * Allocate a new ABD to point to offset off of sabd. It shares the underlying
  * buffer data with sabd. Use abd_put() to free. sabd must not be freed while
  * any derived ABDs exist.
  */
-/* ARGSUSED */
-static inline abd_t *
+static abd_t *
 abd_get_offset_impl(abd_t *sabd, size_t off, size_t size)
 {
-	abd_t *abd;
+	abd_t *abd = NULL;
 
 	abd_verify(sabd);
 	ASSERT3U(off, <=, sabd->abd_size);
@@ -444,60 +307,33 @@ abd_get_offset_impl(abd_t *sabd, size_t off, size_t size)
 		 */
 		abd->abd_flags = ABD_FLAG_LINEAR;
 
-		abd->abd_u.abd_linear.abd_buf =
-		    (char *)sabd->abd_u.abd_linear.abd_buf + off;
+		ABD_LINEAR_BUF(abd) = (char *)ABD_LINEAR_BUF(sabd) + off;
 	} else {
-		size_t new_offset = sabd->abd_u.abd_scatter.abd_offset + off;
-		size_t chunkcnt = abd_scatter_chunkcnt(sabd) -
-		    (new_offset / zfs_abd_chunk_size);
-
-		abd = abd_alloc_struct(chunkcnt);
-
-		/*
-		 * Even if this buf is filesystem metadata, we only track that
-		 * if we own the underlying data buffer, which is not true in
-		 * this case. Therefore, we don't ever use ABD_FLAG_META here.
-		 */
-		abd->abd_flags = 0;
-
-		abd->abd_u.abd_scatter.abd_offset =
-		    new_offset % zfs_abd_chunk_size;
-		abd->abd_u.abd_scatter.abd_chunk_size = zfs_abd_chunk_size;
-
-		/* Copy the scatterlist starting at the correct offset */
-		(void) memcpy(&abd->abd_u.abd_scatter.abd_chunks,
-		    &sabd->abd_u.abd_scatter.abd_chunks[new_offset /
-		    zfs_abd_chunk_size],
-		    chunkcnt * sizeof (void *));
+		abd = abd_get_offset_scatter(sabd, off);
 	}
 
-	if (size == 0)
-		abd->abd_size = sabd->abd_size - off;
-	else
-		abd->abd_size = size;
+	abd->abd_size = size;
 	abd->abd_parent = sabd;
 	zfs_refcount_create(&abd->abd_children);
 	(void) zfs_refcount_add_many(&sabd->abd_children, abd->abd_size, abd);
-
 	return (abd);
 }
 
 abd_t *
 abd_get_offset(abd_t *sabd, size_t off)
 {
-
-	return (abd_get_offset_impl(sabd, off, 0));
+	size_t size = sabd->abd_size > off ? sabd->abd_size - off : 0;
+	VERIFY3U(size, >, 0);
+	return (abd_get_offset_impl(sabd, off, size));
 }
 
 abd_t *
 abd_get_offset_size(abd_t *sabd, size_t off, size_t size)
 {
 	ASSERT3U(off + size, <=, sabd->abd_size);
-
 	return (abd_get_offset_impl(sabd, off, size));
 }
 
-
 /*
  * Allocate a linear ABD structure for buf. You must free this with abd_put()
  * since the resulting ABD doesn't own its own buffer.
@@ -519,33 +355,12 @@ abd_get_from_buf(void *buf, size_t size)
 	abd->abd_parent = NULL;
 	zfs_refcount_create(&abd->abd_children);
 
-	abd->abd_u.abd_linear.abd_buf = buf;
+	ABD_LINEAR_BUF(abd) = buf;
 
 	return (abd);
 }
 
 /*
- * Free an ABD allocated from abd_get_offset() or abd_get_from_buf(). Will not
- * free the underlying scatterlist or buffer.
- */
-void
-abd_put(abd_t *abd)
-{
-	if (abd == NULL)
-		return;
-	abd_verify(abd);
-	ASSERT(!(abd->abd_flags & ABD_FLAG_OWNER));
-
-	if (abd->abd_parent != NULL) {
-		(void) zfs_refcount_remove_many(&abd->abd_parent->abd_children,
-		    abd->abd_size, abd);
-	}
-
-	zfs_refcount_destroy(&abd->abd_children);
-	abd_free_struct(abd);
-}
-
-/*
  * Get the raw buffer associated with a linear ABD.
  */
 void *
@@ -553,7 +368,7 @@ abd_to_buf(abd_t *abd)
 {
 	ASSERT(abd_is_linear(abd));
 	abd_verify(abd);
-	return (abd->abd_u.abd_linear.abd_buf);
+	return (ABD_LINEAR_BUF(abd));
 }
 
 /*
@@ -574,7 +389,6 @@ abd_borrow_buf(abd_t *abd, size_t n)
 		buf = zio_buf_alloc(n);
 	}
 	(void) zfs_refcount_add_many(&abd->abd_children, n, buf);
-
 	return (buf);
 }
 
@@ -617,148 +431,50 @@ abd_return_buf_copy(abd_t *abd, void *buf, size_t n)
 	abd_return_buf(abd, buf, n);
 }
 
-/*
- * Give this ABD ownership of the buffer that it's storing. Can only be used on
- * linear ABDs which were allocated via abd_get_from_buf(), or ones allocated
- * with abd_alloc_linear() which subsequently released ownership of their buf
- * with abd_release_ownership_of_buf().
- */
-void
-abd_take_ownership_of_buf(abd_t *abd, boolean_t is_metadata)
-{
-	ASSERT(abd_is_linear(abd));
-	ASSERT(!(abd->abd_flags & ABD_FLAG_OWNER));
-	abd_verify(abd);
-
-	abd->abd_flags |= ABD_FLAG_OWNER;
-	if (is_metadata) {
-		abd->abd_flags |= ABD_FLAG_META;
-	}
-
-	ABDSTAT_BUMP(abdstat_linear_cnt);
-	ABDSTAT_INCR(abdstat_linear_data_size, abd->abd_size);
-}
-
 void
 abd_release_ownership_of_buf(abd_t *abd)
 {
 	ASSERT(abd_is_linear(abd));
 	ASSERT(abd->abd_flags & ABD_FLAG_OWNER);
+
+	/*
+	 * abd_free() needs to handle LINEAR_PAGE ABD's specially.
+	 * Since that flag does not survive the
+	 * abd_release_ownership_of_buf() -> abd_get_from_buf() ->
+	 * abd_take_ownership_of_buf() sequence, we don't allow releasing
+	 * these "linear but not zio_[data_]buf_alloc()'ed" ABD's.
+	 */
+	ASSERT(!abd_is_linear_page(abd));
+
 	abd_verify(abd);
 
 	abd->abd_flags &= ~ABD_FLAG_OWNER;
 	/* Disable this flag since we no longer own the data buffer */
 	abd->abd_flags &= ~ABD_FLAG_META;
 
-	ABDSTAT_BUMPDOWN(abdstat_linear_cnt);
-	ABDSTAT_INCR(abdstat_linear_data_size, -(int)abd->abd_size);
+	abd_update_linear_stats(abd, ABDSTAT_DECR);
 }
 
-struct abd_iter {
-	abd_t		*iter_abd;	/* ABD being iterated through */
-	size_t		iter_pos;	/* position (relative to abd_offset) */
-	void		*iter_mapaddr;	/* addr corresponding to iter_pos */
-	size_t		iter_mapsize;	/* length of data valid at mapaddr */
-};
-
-static inline size_t
-abd_iter_scatter_chunk_offset(struct abd_iter *aiter)
-{
-	ASSERT(!abd_is_linear(aiter->iter_abd));
-	return ((aiter->iter_abd->abd_u.abd_scatter.abd_offset +
-	    aiter->iter_pos) % zfs_abd_chunk_size);
-}
-
-static inline size_t
-abd_iter_scatter_chunk_index(struct abd_iter *aiter)
-{
-	ASSERT(!abd_is_linear(aiter->iter_abd));
-	return ((aiter->iter_abd->abd_u.abd_scatter.abd_offset +
-	    aiter->iter_pos) / zfs_abd_chunk_size);
-}
 
 /*
- * Initialize the abd_iter.
+ * Give this ABD ownership of the buffer that it's storing. Can only be used on
+ * linear ABDs which were allocated via abd_get_from_buf(), or ones allocated
+ * with abd_alloc_linear() which subsequently released ownership of their buf
+ * with abd_release_ownership_of_buf().
  */
-static void
-abd_iter_init(struct abd_iter *aiter, abd_t *abd)
+void
+abd_take_ownership_of_buf(abd_t *abd, boolean_t is_metadata)
 {
+	ASSERT(abd_is_linear(abd));
+	ASSERT(!(abd->abd_flags & ABD_FLAG_OWNER));
 	abd_verify(abd);
-	aiter->iter_abd = abd;
-	aiter->iter_pos = 0;
-	aiter->iter_mapaddr = NULL;
-	aiter->iter_mapsize = 0;
-}
-
-/*
- * Advance the iterator by a certain amount. Cannot be called when a chunk is
- * in use. This can be safely called when the aiter has already exhausted, in
- * which case this does nothing.
- */
-static void
-abd_iter_advance(struct abd_iter *aiter, size_t amount)
-{
-	ASSERT3P(aiter->iter_mapaddr, ==, NULL);
-	ASSERT0(aiter->iter_mapsize);
-
-	/* There's nothing left to advance to, so do nothing */
-	if (aiter->iter_pos == aiter->iter_abd->abd_size)
-		return;
 
-	aiter->iter_pos += amount;
-}
-
-/*
- * Map the current chunk into aiter. This can be safely called when the aiter
- * has already exhausted, in which case this does nothing.
- */
-static void
-abd_iter_map(struct abd_iter *aiter)
-{
-	void *paddr;
-	size_t offset = 0;
-
-	ASSERT3P(aiter->iter_mapaddr, ==, NULL);
-	ASSERT0(aiter->iter_mapsize);
-
-	/* Panic if someone has changed zfs_abd_chunk_size */
-	IMPLY(!abd_is_linear(aiter->iter_abd), zfs_abd_chunk_size ==
-	    aiter->iter_abd->abd_u.abd_scatter.abd_chunk_size);
-
-	/* There's nothing left to iterate over, so do nothing */
-	if (aiter->iter_pos == aiter->iter_abd->abd_size)
-		return;
-
-	if (abd_is_linear(aiter->iter_abd)) {
-		offset = aiter->iter_pos;
-		aiter->iter_mapsize = aiter->iter_abd->abd_size - offset;
-		paddr = aiter->iter_abd->abd_u.abd_linear.abd_buf;
-	} else {
-		size_t index = abd_iter_scatter_chunk_index(aiter);
-		offset = abd_iter_scatter_chunk_offset(aiter);
-		aiter->iter_mapsize = MIN(zfs_abd_chunk_size - offset,
-		    aiter->iter_abd->abd_size - aiter->iter_pos);
-		paddr = aiter->iter_abd->abd_u.abd_scatter.abd_chunks[index];
+	abd->abd_flags |= ABD_FLAG_OWNER;
+	if (is_metadata) {
+		abd->abd_flags |= ABD_FLAG_META;
 	}
-	aiter->iter_mapaddr = (char *)paddr + offset;
-}
-
-/*
- * Unmap the current chunk from aiter. This can be safely called when the aiter
- * has already exhausted, in which case this does nothing.
- */
-static void
-abd_iter_unmap(struct abd_iter *aiter)
-{
-	/* There's nothing left to unmap, so do nothing */
-	if (aiter->iter_pos == aiter->iter_abd->abd_size)
-		return;
-
-	ASSERT3P(aiter->iter_mapaddr, !=, NULL);
-	ASSERT3U(aiter->iter_mapsize, >, 0);
 
-	aiter->iter_mapaddr = NULL;
-	aiter->iter_mapsize = 0;
+	abd_update_linear_stats(abd, ABDSTAT_INCR);
 }
 
 int
@@ -987,6 +703,7 @@ abd_raidz_gen_iterate(abd_t **cabds, abd_t *dabd,
 	struct abd_iter caiters[3];
 	struct abd_iter daiter = {0};
 	void *caddrs[3];
+	unsigned long flags = 0;
 
 	ASSERT3U(parity, <=, 3);
 
@@ -998,7 +715,7 @@ abd_raidz_gen_iterate(abd_t **cabds, abd_t *dabd,
 
 	ASSERT3S(dsize, >=, 0);
 
-	critical_enter();
+	abd_enter_critical(flags);
 	while (csize > 0) {
 		len = csize;
 
@@ -1010,11 +727,14 @@ abd_raidz_gen_iterate(abd_t **cabds, abd_t *dabd,
 			caddrs[i] = caiters[i].iter_mapaddr;
 		}
 
+
 		switch (parity) {
 			case 3:
 				len = MIN(caiters[2].iter_mapsize, len);
+				/* falls through */
 			case 2:
 				len = MIN(caiters[1].iter_mapsize, len);
+				/* falls through */
 			case 1:
 				len = MIN(caiters[0].iter_mapsize, len);
 		}
@@ -1055,7 +775,7 @@ abd_raidz_gen_iterate(abd_t **cabds, abd_t *dabd,
 		ASSERT3S(dsize, >=, 0);
 		ASSERT3S(csize, >=, 0);
 	}
-	critical_exit();
+	abd_exit_critical(flags);
 }
 
 /*
@@ -1080,6 +800,7 @@ abd_raidz_rec_iterate(abd_t **cabds, abd_t **tabds,
 	struct abd_iter citers[3];
 	struct abd_iter xiters[3];
 	void *caddrs[3], *xaddrs[3];
+	unsigned long flags = 0;
 
 	ASSERT3U(parity, <=, 3);
 
@@ -1088,7 +809,7 @@ abd_raidz_rec_iterate(abd_t **cabds, abd_t **tabds,
 		abd_iter_init(&xiters[i], tabds[i]);
 	}
 
-	critical_enter();
+	abd_enter_critical(flags);
 	while (tsize > 0) {
 
 		for (i = 0; i < parity; i++) {
@@ -1103,9 +824,11 @@ abd_raidz_rec_iterate(abd_t **cabds, abd_t **tabds,
 			case 3:
 				len = MIN(xiters[2].iter_mapsize, len);
 				len = MIN(citers[2].iter_mapsize, len);
+				/* falls through */
 			case 2:
 				len = MIN(xiters[1].iter_mapsize, len);
 				len = MIN(citers[1].iter_mapsize, len);
+				/* falls through */
 			case 1:
 				len = MIN(xiters[0].iter_mapsize, len);
 				len = MIN(citers[0].iter_mapsize, len);
@@ -1130,5 +853,5 @@ abd_raidz_rec_iterate(abd_t **cabds, abd_t **tabds,
 		tsize -= len;
 		ASSERT3S(tsize, >=, 0);
 	}
-	critical_exit();
+	abd_exit_critical(flags);
 }
diff --git a/module/zfs/vdev_indirect.c b/module/zfs/vdev_indirect.c
index 078ba8bab..bfae40b6d 100644
--- a/module/zfs/vdev_indirect.c
+++ b/module/zfs/vdev_indirect.c
@@ -1638,7 +1638,7 @@ vdev_indirect_splits_damage(indirect_vsd_t *iv, zio_t *zio)
 			if (ic->ic_data == NULL)
 				continue;
 
-			abd_zero(ic->ic_data, ic->ic_data->abd_size);
+			abd_zero(ic->ic_data, abd_get_size(ic->ic_data));
 		}
 
 		iv->iv_attempts_max *= 2;