DLPX-44812 integrate EP-220 large memory scalability

1 files changed, 1008 insertions, 0 deletions

diff --git a/module/zfs/abd.c b/module/zfs/abd.c
new file mode 100644
index 000000000..9fa4a5d43
--- /dev/null
+++ b/module/zfs/abd.c
@@ -0,0 +1,1008 @@
+/*
+ * 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 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 weren't using segkpm (see next point) we
+ *      wouldn't need to worry about finding a contiguous address range.
+ *
+ *  (3) Use of segkpm will avoid the need for map / unmap / TLB shootdown costs
+ *      on each ABD access. (If segkpm isn't available then we use all linear
+ *      ABDs to avoid this penalty.) See seg_kpm.c for more details.
+ *
+ * It is possible to make all ABDs linear by setting zfs_abd_scatter_enabled to
+ * B_FALSE. However, it is not possible to use scattered ABDs if segkpm is not
+ * available, which is the case on all 32-bit systems and any 64-bit systems
+ * where kpm_enable is turned off.
+ *
+ * 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>
+
+#ifndef KMC_NOTOUCH
+#define	KMC_NOTOUCH	0
+#endif
+
+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)
+
+/* see block comment above for description */
+int zfs_abd_scatter_enabled = B_TRUE;
+
+
+#ifdef _KERNEL
+static kstat_t *abd_ksp;
+
+static struct page *
+abd_alloc_chunk(void)
+{
+	struct page *c = alloc_page(kmem_flags_convert(KM_SLEEP));
+	ASSERT3P(c, !=, NULL);
+	return (c);
+}
+
+static void
+abd_free_chunk(struct page *c)
+{
+	__free_pages(c, 0);
+}
+
+static void *
+abd_map_chunk(struct page *c)
+{
+	/*
+	 * Use of segkpm means we don't care if this is mapped S_READ or S_WRITE
+	 * but S_WRITE is conceptually more accurate.
+	 */
+	return (kmap(c));
+}
+
+static void
+abd_unmap_chunk(struct page *c)
+{
+	kunmap(c);
+}
+
+void
+abd_init(void)
+{
+	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;
+	}
+}
+
+#else
+
+struct page;
+#define	kpm_enable			1
+#define	abd_alloc_chunk() \
+	((struct page *)kmem_alloc(PAGESIZE, KM_SLEEP))
+#define	abd_free_chunk(chunk) 		kmem_free(chunk, PAGESIZE)
+#define	abd_map_chunk(chunk)		((void *)chunk)
+static void
+abd_unmap_chunk(struct page *c)
+{
+}
+
+void
+abd_init(void)
+{
+}
+
+void
+abd_fini(void)
+{
+}
+
+#endif /* _KERNEL */
+
+static inline size_t
+abd_chunkcnt_for_bytes(size_t size)
+{
+	return (P2ROUNDUP(size, PAGESIZE) / PAGESIZE);
+}
+
+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));
+	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;
+
+		ASSERT3U(abd->abd_u.abd_scatter.abd_offset, <, PAGESIZE);
+		n = abd_scatter_chunkcnt(abd);
+		for (i = 0; i < n; i++) {
+			ASSERT3P(
+			    abd->abd_u.abd_scatter.abd_chunks[i], !=, NULL);
+		}
+	}
+}
+
+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)
+{
+	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);
+}
+
+/*
+ * 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)
+{
+	int i;
+	size_t n;
+	abd_t *abd;
+
+	if (!zfs_abd_scatter_enabled)
+		return (abd_alloc_linear(size, is_metadata));
+
+	VERIFY3U(size, <=, SPA_MAXBLOCKSIZE);
+
+	n = abd_chunkcnt_for_bytes(size);
+	abd = abd_alloc_struct(n);
+
+	abd->abd_flags = ABD_FLAG_OWNER;
+	if (is_metadata) {
+		abd->abd_flags |= ABD_FLAG_META;
+	}
+	abd->abd_size = size;
+	abd->abd_parent = NULL;
+	refcount_create(&abd->abd_children);
+
+	abd->abd_u.abd_scatter.abd_offset = 0;
+	abd->abd_u.abd_scatter.abd_chunk_size = PAGESIZE;
+
+	for (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 * PAGESIZE - size);
+
+	return (abd);
+}
+
+static void
+abd_free_scatter(abd_t *abd)
+{
+	size_t n = abd_scatter_chunkcnt(abd);
+	int i;
+
+	for (i = 0; i < n; i++) {
+		abd_free_chunk(abd->abd_u.abd_scatter.abd_chunks[i]);
+	}
+
+	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 * 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(0);
+
+	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;
+	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->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);
+	}
+
+	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)) {
+		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_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.
+ */
+abd_t *
+abd_get_offset(abd_t *sabd, size_t off)
+{
+	abd_t *abd;
+
+	abd_verify(sabd);
+	ASSERT3U(off, <=, sabd->abd_size);
+
+	if (abd_is_linear(sabd)) {
+		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 = ABD_FLAG_LINEAR;
+
+		abd->abd_u.abd_linear.abd_buf =
+		    (char *)sabd->abd_u.abd_linear.abd_buf + off;
+	} else {
+		size_t new_offset = sabd->abd_u.abd_scatter.abd_offset + off;
+		size_t chunkcnt = abd_scatter_chunkcnt(sabd) -
+		    (new_offset / PAGESIZE);
+
+		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 % PAGESIZE;
+		abd->abd_u.abd_scatter.abd_chunk_size = PAGESIZE;
+
+		/* 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 / PAGESIZE],
+		    chunkcnt * sizeof (void *));
+	}
+
+	abd->abd_size = sabd->abd_size - off;
+	abd->abd_parent = sabd;
+	refcount_create(&abd->abd_children);
+	(void) refcount_add_many(&sabd->abd_children, abd->abd_size, abd);
+
+	return (abd);
+}
+
+/*
+ * 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(0);
+
+	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;
+	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) refcount_remove_many(&abd->abd_parent->abd_children,
+		    abd->abd_size, abd);
+	}
+
+	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) 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) 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_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 {
+	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) % PAGESIZE);
+}
+
+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) / PAGESIZE);
+}
+
+/*
+ * Initialize the abd_iter.
+ */
+static 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;
+}
+
+/*
+ * 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);
+
+	/* 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 = PAGESIZE - offset;
+		paddr = abd_map_chunk(
+			aiter->iter_abd->abd_u.abd_scatter.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.
+ */
+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 */
+		size_t index = abd_iter_scatter_chunk_index(aiter);
+		abd_unmap_chunk(
+		    aiter->iter_abd->abd_u.abd_scatter.abd_chunks[index]);
+	}
+
+	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);
+	abd_iter_advance(&aiter, off);
+
+	while (size > 0) {
+		size_t len;
+		abd_iter_map(&aiter);
+
+		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);
+	abd_iter_init(&saiter, sabd);
+	abd_iter_advance(&daiter, doff);
+	abd_iter_advance(&saiter, soff);
+
+	while (size > 0) {
+		size_t dlen, slen, len;
+		abd_iter_map(&daiter);
+		abd_iter_map(&saiter);
+
+		dlen = MIN(daiter.iter_mapsize, size);
+		slen = MIN(saiter.iter_mapsize, size);
+		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));
+}
+
+
+#if defined(_KERNEL) && defined(HAVE_SPL)
+/* Tunable Parameters */
+module_param(zfs_abd_scatter_enabled, int, 0644);
+MODULE_PARM_DESC(zfs_abd_scatter_enabled,
+	"Toggle whether ABD allocations must be linear.");
+#endif