BRT: Fix FICLONE/FICLONERANGE shortened copy

On Linux the ioctl_ficlonerange() and ioctl_ficlone() system calls
are expected to either fully clone the specified range or return an
error.  The range may be for an entire file.  While internally ZFS
supports cloning partial ranges there's no way to return the length
cloned to the caller so we need to make this all or nothing.

As part of this change support for the REMAP_FILE_CAN_SHORTEN flag
has been added.  When REMAP_FILE_CAN_SHORTEN is set zfs_clone_range()
will return a shortened range when encountering pending dirty records.
When it's clear zfs_clone_range() will block and wait for the records
to be written out allowing the blocks to be cloned.

Furthermore, the file range lock is held over the region being cloned
to prevent it from being modified while cloning.  This doesn't quite
provide an atomic semantics since if an error is encountered only a
portion of the range may be cloned.  This will be converted to an
error if REMAP_FILE_CAN_SHORTEN was not provided and returned to the
caller.  However, the destination file range is left in an undefined
state.

A test case has been added which exercises this functionality by
verifying that `cp --reflink=never|auto|always` works correctly.

Reviewed-by: Alexander Motin <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #15728
Closes #15842

13 files changed, 243 insertions, 39 deletions

diff --git a/include/os/freebsd/zfs/sys/zfs_vfsops_os.h b/include/os/freebsd/zfs/sys/zfs_vfsops_os.h
index e7ebcccbe..7f0f24325 100644
--- a/include/os/freebsd/zfs/sys/zfs_vfsops_os.h
+++ b/include/os/freebsd/zfs/sys/zfs_vfsops_os.h
@@ -285,7 +285,6 @@ typedef struct zfid_long {
 #define	LONG_FID_LEN	(sizeof (zfid_long_t) - sizeof (uint16_t))
 
 extern int zfs_super_owner;
-extern int zfs_bclone_enabled;
 
 extern void zfs_init(void);
 extern void zfs_fini(void);
diff --git a/include/os/linux/zfs/sys/zfs_vfsops_os.h b/include/os/linux/zfs/sys/zfs_vfsops_os.h
index 220466550..b4d5db21f 100644
--- a/include/os/linux/zfs/sys/zfs_vfsops_os.h
+++ b/include/os/linux/zfs/sys/zfs_vfsops_os.h
@@ -45,8 +45,6 @@ extern "C" {
 typedef struct zfsvfs zfsvfs_t;
 struct znode;
 
-extern int zfs_bclone_enabled;
-
 /*
  * This structure emulates the vfs_t from other platforms.  It's purpose
  * is to facilitate the handling of mount options and minimize structural
diff --git a/include/sys/zfs_vnops.h b/include/sys/zfs_vnops.h
index 5da103f17..e60b99bed 100644
--- a/include/sys/zfs_vnops.h
+++ b/include/sys/zfs_vnops.h
@@ -24,8 +24,11 @@
 
 #ifndef	_SYS_FS_ZFS_VNOPS_H
 #define	_SYS_FS_ZFS_VNOPS_H
+
 #include <sys/zfs_vnops_os.h>
 
+extern int zfs_bclone_enabled;
+
 extern int zfs_fsync(znode_t *, int, cred_t *);
 extern int zfs_read(znode_t *, zfs_uio_t *, int, cred_t *);
 extern int zfs_write(znode_t *, zfs_uio_t *, int, cred_t *);
diff --git a/man/man4/zfs.4 b/man/man4/zfs.4
index 47471a805..30c168253 100644
--- a/man/man4/zfs.4
+++ b/man/man4/zfs.4
@@ -1159,6 +1159,15 @@ Enable the experimental block cloning feature.
 If this setting is 0, then even if feature@block_cloning is enabled,
 attempts to clone blocks will act as though the feature is disabled.
 .
+.It Sy zfs_bclone_wait_dirty Ns = Ns Sy 0 Ns | Ns 1 Pq int
+When set to 1 the FICLONE and FICLONERANGE ioctls wait for dirty data to be
+written to disk.
+This allows the clone operation to reliably succeed when a file is
+modified and then immediately cloned.
+For small files this may be slower than making a copy of the file.
+Therefore, this setting defaults to 0 which causes a clone operation to
+immediately fail when encountering a dirty block.
+.
 .It Sy zfs_blake3_impl Ns = Ns Sy fastest Pq string
 Select a BLAKE3 implementation.
 .Pp
diff --git a/module/os/freebsd/zfs/zfs_vfsops.c b/module/os/freebsd/zfs/zfs_vfsops.c
index f2d539103..a972c720d 100644
--- a/module/os/freebsd/zfs/zfs_vfsops.c
+++ b/module/os/freebsd/zfs/zfs_vfsops.c
@@ -89,10 +89,6 @@ int zfs_debug_level;
 SYSCTL_INT(_vfs_zfs, OID_AUTO, debug, CTLFLAG_RWTUN, &zfs_debug_level, 0,
 	"Debug level");
 
-int zfs_bclone_enabled = 1;
-SYSCTL_INT(_vfs_zfs, OID_AUTO, bclone_enabled, CTLFLAG_RWTUN,
-	&zfs_bclone_enabled, 0, "Enable block cloning");
-
 struct zfs_jailparam {
 	int mount_snapshot;
 };
diff --git a/module/os/linux/zfs/zfs_vnops_os.c b/module/os/linux/zfs/zfs_vnops_os.c
index b7b89b8af..a32307c39 100644
--- a/module/os/linux/zfs/zfs_vnops_os.c
+++ b/module/os/linux/zfs/zfs_vnops_os.c
@@ -4255,9 +4255,4 @@ EXPORT_SYMBOL(zfs_map);
 /* CSTYLED */
 module_param(zfs_delete_blocks, ulong, 0644);
 MODULE_PARM_DESC(zfs_delete_blocks, "Delete files larger than N blocks async");
-
-/* CSTYLED */
-module_param(zfs_bclone_enabled, uint, 0644);
-MODULE_PARM_DESC(zfs_bclone_enabled, "Enable block cloning");
-
 #endif
diff --git a/module/os/linux/zfs/zpl_file_range.c b/module/os/linux/zfs/zpl_file_range.c
index 73476ff40..3065d54fa 100644
--- a/module/os/linux/zfs/zpl_file_range.c
+++ b/module/os/linux/zfs/zpl_file_range.c
@@ -31,8 +31,6 @@
 #include <sys/zfs_vnops.h>
 #include <sys/zfeature.h>
 
-int zfs_bclone_enabled = 1;
-
 /*
  * Clone part of a file via block cloning.
  *
@@ -40,7 +38,7 @@ int zfs_bclone_enabled = 1;
  * care of that depending on how it was called.
  */
 static ssize_t
-__zpl_clone_file_range(struct file *src_file, loff_t src_off,
+zpl_clone_file_range_impl(struct file *src_file, loff_t src_off,
     struct file *dst_file, loff_t dst_off, size_t len)
 {
 	struct inode *src_i = file_inode(src_file);
@@ -96,11 +94,12 @@ zpl_copy_file_range(struct file *src_file, loff_t src_off,
 {
 	ssize_t ret;
 
+	/* Flags is reserved for future extensions and must be zero. */
 	if (flags != 0)
 		return (-EINVAL);
 
-	/* Try to do it via zfs_clone_range() */
-	ret = __zpl_clone_file_range(src_file, src_off,
+	/* Try to do it via zfs_clone_range() and allow shortening. */
+	ret = zpl_clone_file_range_impl(src_file, src_off,
 	    dst_file, dst_off, len);
 
 #ifdef HAVE_VFS_GENERIC_COPY_FILE_RANGE
@@ -137,6 +136,11 @@ zpl_copy_file_range(struct file *src_file, loff_t src_off,
  * FIDEDUPERANGE is for turning a non-clone into a clone, that is, compare the
  * range in both files and if they're the same, arrange for them to be backed
  * by the same storage.
+ *
+ * REMAP_FILE_CAN_SHORTEN lets us know we can clone less than the given range
+ * if we want. It's designed for filesystems that may need to shorten the
+ * length for alignment, EOF, or any other requirement. ZFS may shorten the
+ * request when there is outstanding dirty data which hasn't been written.
  */
 loff_t
 zpl_remap_file_range(struct file *src_file, loff_t src_off,
@@ -145,24 +149,21 @@ zpl_remap_file_range(struct file *src_file, loff_t src_off,
 	if (flags & ~(REMAP_FILE_DEDUP | REMAP_FILE_CAN_SHORTEN))
 		return (-EINVAL);
 
-	/*
-	 * REMAP_FILE_CAN_SHORTEN lets us know we can clone less than the given
-	 * range if we want. Its designed for filesystems that make data past
-	 * EOF available, and don't want it to be visible in both files. ZFS
-	 * doesn't do that, so we just turn the flag off.
-	 */
-	flags &= ~REMAP_FILE_CAN_SHORTEN;
-
+	/* No support for dedup yet */
 	if (flags & REMAP_FILE_DEDUP)
-		/* No support for dedup yet */
 		return (-EOPNOTSUPP);
 
 	/* Zero length means to clone everything to the end of the file */
 	if (len == 0)
 		len = i_size_read(file_inode(src_file)) - src_off;
 
-	return (__zpl_clone_file_range(src_file, src_off,
-	    dst_file, dst_off, len));
+	ssize_t ret = zpl_clone_file_range_impl(src_file, src_off,
+	    dst_file, dst_off, len);
+
+	if (!(flags & REMAP_FILE_CAN_SHORTEN) && ret >= 0 && ret != len)
+		ret = -EINVAL;
+
+	return (ret);
 }
 #endif /* HAVE_VFS_REMAP_FILE_RANGE */
 
@@ -179,8 +180,14 @@ zpl_clone_file_range(struct file *src_file, loff_t src_off,
 	if (len == 0)
 		len = i_size_read(file_inode(src_file)) - src_off;
 
-	return (__zpl_clone_file_range(src_file, src_off,
-	    dst_file, dst_off, len));
+	/* The entire length must be cloned or this is an error. */
+	ssize_t ret = zpl_clone_file_range_impl(src_file, src_off,
+	    dst_file, dst_off, len);
+
+	if (ret >= 0 && ret != len)
+		ret = -EINVAL;
+
+	return (ret);
 }
 #endif /* HAVE_VFS_CLONE_FILE_RANGE || HAVE_VFS_FILE_OPERATIONS_EXTEND */
 
@@ -214,8 +221,7 @@ zpl_ioctl_ficlone(struct file *dst_file, void *arg)
 
 	size_t len = i_size_read(file_inode(src_file));
 
-	ssize_t ret =
-	    __zpl_clone_file_range(src_file, 0, dst_file, 0, len);
+	ssize_t ret = zpl_clone_file_range_impl(src_file, 0, dst_file, 0, len);
 
 	fput(src_file);
 
@@ -253,7 +259,7 @@ zpl_ioctl_ficlonerange(struct file *dst_file, void __user *arg)
 	if (len == 0)
 		len = i_size_read(file_inode(src_file)) - fcr.fcr_src_offset;
 
-	ssize_t ret = __zpl_clone_file_range(src_file, fcr.fcr_src_offset,
+	ssize_t ret = zpl_clone_file_range_impl(src_file, fcr.fcr_src_offset,
 	    dst_file, fcr.fcr_dest_offset, len);
 
 	fput(src_file);
diff --git a/module/zfs/zfs_vnops.c b/module/zfs/zfs_vnops.c
index c8ff7b643..7f39ad6fc 100644
--- a/module/zfs/zfs_vnops.c
+++ b/module/zfs/zfs_vnops.c
@@ -58,6 +58,26 @@
 #include <sys/zfs_vfsops.h>
 #include <sys/zfs_znode.h>
 
+/*
+ * Enable the experimental block cloning feature.  If this setting is 0, then
+ * even if feature@block_cloning is enabled, attempts to clone blocks will act
+ * as though the feature is disabled.
+ */
+int zfs_bclone_enabled = 1;
+
+/*
+ * When set zfs_clone_range() waits for dirty data to be written to disk.
+ * This allows the clone operation to reliably succeed when a file is modified
+ * and then immediately cloned. For small files this may be slower than making
+ * a copy of the file and is therefore not the default.  However, in certain
+ * scenarios this behavior may be desirable so a tunable is provided.
+ */
+static int zfs_bclone_wait_dirty = 0;
+
+/*
+ * Maximum bytes to read per chunk in zfs_read().
+ */
+static uint64_t zfs_vnops_read_chunk_size = 1024 * 1024;
 
 int
 zfs_fsync(znode_t *zp, int syncflag, cred_t *cr)
@@ -182,8 +202,6 @@ zfs_access(znode_t *zp, int mode, int flag, cred_t *cr)
 	return (error);
 }
 
-static uint64_t zfs_vnops_read_chunk_size = 1024 * 1024; /* Tunable */
-
 /*
  * Read bytes from specified file into supplied buffer.
  *
@@ -1049,6 +1067,7 @@ zfs_clone_range(znode_t *inzp, uint64_t *inoffp, znode_t *outzp,
 	size_t		maxblocks, nbps;
 	uint_t		inblksz;
 	uint64_t	clear_setid_bits_txg = 0;
+	uint64_t	last_synced_txg = 0;
 
 	inoff = *inoffp;
 	outoff = *outoffp;
@@ -1287,15 +1306,23 @@ zfs_clone_range(znode_t *inzp, uint64_t *inoffp, znode_t *outzp,
 		}
 
 		nbps = maxblocks;
+		last_synced_txg = spa_last_synced_txg(dmu_objset_spa(inos));
 		error = dmu_read_l0_bps(inos, inzp->z_id, inoff, size, bps,
 		    &nbps);
 		if (error != 0) {
 			/*
 			 * If we are trying to clone a block that was created
-			 * in the current transaction group, error will be
-			 * EAGAIN here, which we can just return to the caller
-			 * so it can fallback if it likes.
+			 * in the current transaction group, the error will be
+			 * EAGAIN here.  Based on zfs_bclone_wait_dirty either
+			 * return a shortened range to the caller so it can
+			 * fallback, or wait for the next TXG and check again.
 			 */
+			if (error == EAGAIN && zfs_bclone_wait_dirty) {
+				txg_wait_synced(dmu_objset_pool(inos),
+				    last_synced_txg + 1);
+				continue;
+			}
+
 			break;
 		}
 
@@ -1517,3 +1544,9 @@ EXPORT_SYMBOL(zfs_clone_range_replay);
 
 ZFS_MODULE_PARAM(zfs_vnops, zfs_vnops_, read_chunk_size, U64, ZMOD_RW,
 	"Bytes to read per chunk");
+
+ZFS_MODULE_PARAM(zfs, zfs_, bclone_enabled, INT, ZMOD_RW,
+	"Enable block cloning");
+
+ZFS_MODULE_PARAM(zfs, zfs_, bclone_wait_dirty, INT, ZMOD_RW,
+	"Wait for dirty blocks when cloning");
diff --git a/tests/runfiles/common.run b/tests/runfiles/common.run
index 7e0990b5d..05e8cdc8f 100644
--- a/tests/runfiles/common.run
+++ b/tests/runfiles/common.run
@@ -631,7 +631,7 @@ tests = ['compress_001_pos', 'compress_002_pos', 'compress_003_pos',
 tags = ['functional', 'compression']
 
 [tests/functional/cp_files]
-tests = ['cp_files_001_pos', 'cp_stress']
+tests = ['cp_files_001_pos', 'cp_files_002_pos', 'cp_stress']
 tags = ['functional', 'cp_files']
 
 [tests/functional/crtime]
diff --git a/tests/test-runner/bin/zts-report.py.in b/tests/test-runner/bin/zts-report.py.in
index ae4aa6275..edfdd47ee 100755
--- a/tests/test-runner/bin/zts-report.py.in
+++ b/tests/test-runner/bin/zts-report.py.in
@@ -176,6 +176,7 @@ if sys.platform.startswith('freebsd'):
         'cli_root/zpool_wait/zpool_wait_trim_cancel': ['SKIP', trim_reason],
         'cli_root/zpool_wait/zpool_wait_trim_flag': ['SKIP', trim_reason],
         'cli_root/zfs_unshare/zfs_unshare_008_pos': ['SKIP', na_reason],
+        'cp_files/cp_files_002_pos': ['SKIP', na_reason],
         'link_count/link_count_001': ['SKIP', na_reason],
         'casenorm/mixed_create_failure': ['FAIL', 13215],
         'mmap/mmap_sync_001_pos': ['SKIP', na_reason],
@@ -312,6 +313,7 @@ elif sys.platform.startswith('linux'):
             ['SKIP', cfr_reason],
         'cli_root/zfs_rename/zfs_rename_002_pos': ['FAIL', known_reason],
         'cli_root/zpool_reopen/zpool_reopen_003_pos': ['FAIL', known_reason],
+        'cp_files/cp_files_002_pos': ['SKIP', cfr_reason],
         'fault/auto_online_002_pos': ['FAIL', 11889],
         'fault/auto_replace_001_pos': ['FAIL', 14851],
         'fault/auto_spare_002_pos': ['FAIL', 11889],
diff --git a/tests/zfs-tests/include/tunables.cfg b/tests/zfs-tests/include/tunables.cfg
index e4e380aa7..a619b846d 100644
--- a/tests/zfs-tests/include/tunables.cfg
+++ b/tests/zfs-tests/include/tunables.cfg
@@ -94,6 +94,7 @@ VOL_MODE			vol.mode			zvol_volmode
 VOL_RECURSIVE			vol.recursive			UNSUPPORTED
 VOL_USE_BLK_MQ			UNSUPPORTED			zvol_use_blk_mq
 BCLONE_ENABLED			zfs_bclone_enabled		zfs_bclone_enabled
+BCLONE_WAIT_DIRTY		zfs_bclone_wait_dirty		zfs_bclone_wait_dirty
 XATTR_COMPAT			xattr_compat			zfs_xattr_compat
 ZEVENT_LEN_MAX			zevent.len_max			zfs_zevent_len_max
 ZEVENT_RETAIN_MAX		zevent.retain_max		zfs_zevent_retain_max
diff --git a/tests/zfs-tests/tests/Makefile.am b/tests/zfs-tests/tests/Makefile.am
index 4040e6043..44fa0bf68 100644
--- a/tests/zfs-tests/tests/Makefile.am
+++ b/tests/zfs-tests/tests/Makefile.am
@@ -1394,6 +1394,7 @@ nobase_dist_datadir_zfs_tests_tests_SCRIPTS += \
 	functional/compression/setup.ksh \
 	functional/cp_files/cleanup.ksh \
 	functional/cp_files/cp_files_001_pos.ksh \
+	functional/cp_files/cp_files_002_pos.ksh \
 	functional/cp_files/cp_stress.ksh \
 	functional/cp_files/setup.ksh \
 	functional/crtime/cleanup.ksh \
diff --git a/tests/zfs-tests/tests/functional/cp_files/cp_files_002_pos.ksh b/tests/zfs-tests/tests/functional/cp_files/cp_files_002_pos.ksh
new file mode 100755
index 000000000..60817449a
--- /dev/null
+++ b/tests/zfs-tests/tests/functional/cp_files/cp_files_002_pos.ksh
@@ -0,0 +1,161 @@
+#! /bin/ksh -p
+#
+# 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 https://opensource.org/licenses/CDDL-1.0.
+# 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) 2024 by Lawrence Livermore National Security, LLC.
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/bclone/bclone_common.kshlib
+
+#
+# DESCRIPTION:
+# Verify all cp --reflink modes work with modified file.
+#
+# STRATEGY:
+# 1. Verify "cp --reflink=never|auto|always" behaves as expected.
+#    Two different modes of operation are tested.
+#
+#    a. zfs_bclone_wait_dirty=0: FICLONE and FICLONERANGE fail with EINVAL
+#       when there are dirty blocks which cannot be immediately cloned.
+#       This is the default behavior.
+#
+#    b. zfs_bclone_wait_dirty=1: FICLONE and FICLONERANGE wait for
+#       dirty blocks to be written to disk allowing the clone to succeed.
+#       The downside to this is it may be slow which depending on the
+#       situtation may defeat the point of making a clone.
+#
+
+verify_runnable "global"
+verify_block_cloning
+
+if ! is_linux; then
+	log_unsupported "cp --reflink is a GNU coreutils option"
+fi
+
+function cleanup
+{
+	datasetexists $TESTPOOL/cp-reflink && \
+	    destroy_dataset $$TESTPOOL/cp-reflink -f
+	log_must set_tunable32 BCLONE_WAIT_DIRTY 0
+}
+
+function verify_copy
+{
+	src_cksum=$(sha256digest $1)
+	dst_cksum=$(sha256digest $2)
+
+	if [[ "$src_cksum" != "$dst_cksum" ]]; then
+		log_must ls -l $CP_TESTDIR
+		log_fail "checksum mismatch ($src_cksum != $dst_cksum)"
+	fi
+}
+
+log_assert "Verify all cp --reflink modes work with modified file"
+
+log_onexit cleanup
+
+SRC_FILE=src.data
+DST_FILE=dst.data
+SRC_SIZE=$(($RANDOM % 2048))
+
+# A smaller recordsize is used merely to speed up the test.
+RECORDSIZE=4096
+
+log_must zfs create -o recordsize=$RECORDSIZE $TESTPOOL/cp-reflink
+CP_TESTDIR=$(get_prop mountpoint $TESTPOOL/cp-reflink)
+
+log_must cd $CP_TESTDIR
+
+# Never wait on dirty blocks (zfs_bclone_wait_dirty=0)
+log_must set_tunable32 BCLONE_WAIT_DIRTY 0
+
+for mode in "never" "auto" "always"; do
+	log_note "Checking 'cp --reflink=$mode'"
+
+	# Create a new file and immediately copy it.
+	log_must dd if=/dev/urandom of=$SRC_FILE bs=$RECORDSIZE count=$SRC_SIZE
+
+	if [[ "$mode" == "always" ]]; then
+		log_mustnot cp --reflink=$mode $SRC_FILE $DST_FILE
+		log_must ls -l $CP_TESTDIR
+	else
+		log_must cp --reflink=$mode $SRC_FILE $DST_FILE
+		verify_copy $SRC_FILE $DST_FILE
+	fi
+	log_must rm -f $DST_FILE
+
+	# Append to an existing file and immediately copy it.
+	sync_pool $TESTPOOL
+	log_must dd if=/dev/urandom of=$SRC_FILE bs=$RECORDSIZE seek=$SRC_SIZE \
+	    count=1 conv=notrunc
+	if [[ "$mode" == "always" ]]; then
+		log_mustnot cp --reflink=$mode $SRC_FILE $DST_FILE
+		log_must ls -l $CP_TESTDIR
+	else
+		log_must cp --reflink=$mode $SRC_FILE $DST_FILE
+		verify_copy $SRC_FILE $DST_FILE
+	fi
+	log_must rm -f $DST_FILE
+
+	# Overwrite a random range of an existing file and immediately copy it.
+	sync_pool $TESTPOOL
+	log_must dd if=/dev/urandom of=$SRC_FILE bs=$((RECORDSIZE / 2)) \
+            seek=$(($RANDOM % $SRC_SIZE)) count=$(($RANDOM % 16)) conv=notrunc
+	if [[ "$mode" == "always" ]]; then
+		log_mustnot cp --reflink=$mode $SRC_FILE $DST_FILE
+		log_must ls -l $CP_TESTDIR
+	else
+		log_must cp --reflink=$mode $SRC_FILE $DST_FILE
+		verify_copy $SRC_FILE $DST_FILE
+	fi
+	log_must rm -f $SRC_FILE $DST_FILE
+done
+
+# Wait on dirty blocks (zfs_bclone_wait_dirty=1)
+log_must set_tunable32 BCLONE_WAIT_DIRTY 1
+
+for mode in "never" "auto" "always"; do
+	log_note "Checking 'cp --reflink=$mode'"
+
+	# Create a new file and immediately copy it.
+	log_must dd if=/dev/urandom of=$SRC_FILE bs=$RECORDSIZE count=$SRC_SIZE
+	log_must cp --reflink=$mode $SRC_FILE $DST_FILE
+	verify_copy $SRC_FILE $DST_FILE
+	log_must rm -f $DST_FILE
+
+	# Append to an existing file and immediately copy it.
+	log_must dd if=/dev/urandom of=$SRC_FILE bs=$RECORDSIZE seek=$SRC_SIZE \
+	    count=1 conv=notrunc
+	log_must cp --reflink=$mode $SRC_FILE $DST_FILE
+	verify_copy $SRC_FILE $DST_FILE
+	log_must rm -f $DST_FILE
+
+	# Overwrite a random range of an existing file and immediately copy it.
+	log_must dd if=/dev/urandom of=$SRC_FILE bs=$((RECORDSIZE / 2)) \
+            seek=$(($RANDOM % $SRC_SIZE)) count=$(($RANDOM % 16)) conv=notrunc
+	log_must cp --reflink=$mode $SRC_FILE $DST_FILE
+	verify_copy $SRC_FILE $DST_FILE
+	log_must rm -f $SRC_FILE $DST_FILE
+done
+
+log_pass