diff mbox series

[v23.2,1/1] xfs: test xfs_scrub phase 6 media error reporting

Message ID Y2NAwmtq+ey1d2mF@magnolia (mailing list archive)
State New, archived
Headers show
Series [v23.2,1/1] xfs: test xfs_scrub phase 6 media error reporting | expand

Commit Message

Darrick J. Wong Nov. 3, 2022, 4:17 a.m. UTC 
From: Darrick J. Wong <djwong@kernel.org>

Add new helpers to dmerror to provide for marking selected ranges
totally bad -- both reads and writes will fail.  Create a new test for
xfs_scrub to check that it reports media errors in data files correctly.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
v23.2: fix missing group name, transform awk program style fully
---
 common/dmerror    |  147 ++++++++++++++++++++++++++++++++++++++++++++++++++
 common/xfs        |    9 +++
 tests/xfs/747     |  155 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 tests/xfs/747.out |   12 ++++
 4 files changed, 323 insertions(+)
 create mode 100755 tests/xfs/747
 create mode 100644 tests/xfs/747.out

Comments

Zorro Lang Nov. 3, 2022, 5 a.m. UTC | #1 
On Wed, Nov 02, 2022 at 09:17:06PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> Add new helpers to dmerror to provide for marking selected ranges
> totally bad -- both reads and writes will fail.  Create a new test for
> xfs_scrub to check that it reports media errors in data files correctly.
> 
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
> v23.2: fix missing group name, transform awk program style fully
> ---

This version is good to me,
Reviewed-by: Zorro Lang <zlang@redhat.com>

>  common/dmerror    |  147 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  common/xfs        |    9 +++
>  tests/xfs/747     |  155 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  tests/xfs/747.out |   12 ++++
>  4 files changed, 323 insertions(+)
>  create mode 100755 tests/xfs/747
>  create mode 100644 tests/xfs/747.out
> 
> diff --git a/common/dmerror b/common/dmerror
> index 54122b12ea..3494b6dd3b 100644
> --- a/common/dmerror
> +++ b/common/dmerror
> @@ -250,3 +250,150 @@ _dmerror_load_working_table()
>  	[ $load_res -ne 0 ] && _fail "dmsetup failed to load error table"
>  	[ $resume_res -ne 0 ] && _fail  "dmsetup resume failed"
>  }
> +
> +# Given a list of (start, length) tuples on stdin, combine adjacent tuples into
> +# larger ones and write the new list to stdout.
> +__dmerror_combine_extents()
> +{
> +	local awk_program='
> +	BEGIN {
> +		start = 0; len = 0;
> +	}
> +	{
> +		if (start + len == $1) {
> +			len += $2;
> +		} else {
> +			if (len > 0)
> +				printf("%d %d\n", start, len);
> +			start = $1;
> +			len = $2;
> +		}
> +	}
> +	END {
> +		if (len > 0)
> +			printf("%d %d\n", start, len);
> +	}'
> +
> +	awk "$awk_program"
> +}
> +
> +# Given a block device, the name of a preferred dm target, the name of an
> +# implied dm target, and a list of (start, len) tuples on stdin, create a new
> +# dm table which maps each of the tuples to the preferred target and all other
> +# areas to the implied dm target.
> +__dmerror_recreate_map()
> +{
> +	local device="$1"
> +	local preferred_tgt="$2"
> +	local implied_tgt="$3"
> +	local size=$(blockdev --getsz "$device")
> +
> +	local awk_program='
> +	BEGIN {
> +		implied_start = 0;
> +	}
> +	{
> +		extent_start = $1;
> +		extent_len = $2;
> +
> +		if (extent_start > size) {
> +			extent_start = size;
> +			extent_len = 0;
> +		} else if (extent_start + extent_len > size) {
> +			extent_len = size - extent_start;
> +		}
> +
> +		if (implied_start < extent_start)
> +			printf("%d %d %s %s %d\n", implied_start,
> +					extent_start - implied_start,
> +					implied_tgt, device, implied_start);
> +		printf("%d %d %s %s %d\n", extent_start, extent_len,
> +				preferred_tgt, device, extent_start);
> +		implied_start = extent_start + extent_len;
> +	}
> +	END {
> +		if (implied_start < size)
> +			printf("%d %d %s %s %d\n", implied_start,
> +					size - implied_start, implied_tgt,
> +					device, implied_start);
> +	}'
> +
> +	awk -v device="$device" -v size=$size -v implied_tgt="$implied_tgt" \
> +		-v preferred_tgt="$preferred_tgt" "$awk_program"
> +}
> +
> +# Update the dm error table so that the range (start, len) maps to the
> +# preferred dm target, overriding anything that maps to the implied dm target.
> +# This assumes that the only desired targets for this dm device are the
> +# preferred and and implied targets.  The fifth argument is the scratch device
> +# that we want to change the table for.
> +__dmerror_change()
> +{
> +	local start="$1"
> +	local len="$2"
> +	local preferred_tgt="$3"
> +	local implied_tgt="$4"
> +	local whichdev="$5"
> +	local old_table
> +	local new_table
> +
> +	case "$whichdev" in
> +	"SCRATCH_DEV"|"")	whichdev="$SCRATCH_DEV";;
> +	"SCRATCH_LOGDEV"|"LOG")	whichdev="$NON_ERROR_LOGDEV";;
> +	"SCRATCH_RTDEV"|"RT")	whichdev="$NON_ERROR_RTDEV";;
> +	esac
> +
> +	case "$whichdev" in
> +	"$SCRATCH_DEV")		old_table="$DMERROR_TABLE";;
> +	"$NON_ERROR_LOGDEV")	old_table="$DMERROR_LOGTABLE";;
> +	"$NON_ERROR_RTDEV")	old_table="$DMERROR_RTTABLE";;
> +	*)
> +		echo "$whichdev: Unknown dmerror device."
> +		return
> +		;;
> +	esac
> +
> +	new_table="$( (echo "$old_table"; echo "$start $len $preferred_tgt") | \
> +		awk -v type="$preferred_tgt" '{if ($3 == type) print $0;}' | \
> +		sort -g | \
> +		__dmerror_combine_extents | \
> +		__dmerror_recreate_map "$whichdev" "$preferred_tgt" \
> +				"$implied_tgt" )"
> +
> +	case "$whichdev" in
> +	"$SCRATCH_DEV")		DMERROR_TABLE="$new_table";;
> +	"$NON_ERROR_LOGDEV")	DMERROR_LOGTABLE="$new_table";;
> +	"$NON_ERROR_RTDEV")	DMERROR_RTTABLE="$new_table";;
> +	esac
> +}
> +
> +# Reset the dm error table to everything ok.  The dm device itself must be
> +# remapped by calling _dmerror_load_error_table.
> +_dmerror_reset_table()
> +{
> +	DMERROR_TABLE="$DMLINEAR_TABLE"
> +	DMERROR_LOGTABLE="$DMLINEAR_LOGTABLE"
> +	DMERROR_RTTABLE="$DMLINEAR_RTTABLE"
> +}
> +
> +# Update the dm error table so that IOs to the given range will return EIO.
> +# The dm device itself must be remapped by calling _dmerror_load_error_table.
> +_dmerror_mark_range_bad()
> +{
> +	local start="$1"
> +	local len="$2"
> +	local dev="$3"
> +
> +	__dmerror_change "$start" "$len" error linear "$dev"
> +}
> +
> +# Update the dm error table so that IOs to the given range will succeed.
> +# The dm device itself must be remapped by calling _dmerror_load_error_table.
> +_dmerror_mark_range_good()
> +{
> +	local start="$1"
> +	local len="$2"
> +	local dev="$3"
> +
> +	__dmerror_change "$start" "$len" linear error "$dev"
> +}
> diff --git a/common/xfs b/common/xfs
> index 8ac1964e9c..f466d2c42f 100644
> --- a/common/xfs
> +++ b/common/xfs
> @@ -218,6 +218,15 @@ _xfs_get_dir_blocksize()
>  	$XFS_INFO_PROG "$fs" | sed -n "s/^naming.*bsize=\([[:digit:]]*\).*/\1/p"
>  }
>  
> +# Decide if this path is a file on the realtime device
> +_xfs_is_realtime_file()
> +{
> +	if [ "$USE_EXTERNAL" != "yes" ] || [ -z "$SCRATCH_RTDEV" ]; then
> +		return 1
> +	fi
> +	$XFS_IO_PROG -c 'stat -v' "$1" | grep -q -w realtime
> +}
> +
>  # Set or clear the realtime status of every supplied path.  The first argument
>  # is either 'data' or 'realtime'.  All other arguments should be paths to
>  # existing directories or empty regular files.
> diff --git a/tests/xfs/747 b/tests/xfs/747
> new file mode 100755
> index 0000000000..847e1e17f8
> --- /dev/null
> +++ b/tests/xfs/747
> @@ -0,0 +1,155 @@
> +#! /bin/bash
> +# SPDX-License-Identifier: GPL-2.0-or-later
> +# Copyright (c) 2022 Oracle.  All Rights Reserved.
> +#
> +# FS QA Test No. 747
> +#
> +# Check xfs_scrub's media scan can actually return diagnostic information for
> +# media errors in file data extents.
> +
> +. ./common/preamble
> +_begin_fstest auto quick scrub eio
> +
> +# Override the default cleanup function.
> +_cleanup()
> +{
> +	cd /
> +	rm -f $tmp.*
> +	_dmerror_cleanup
> +}
> +
> +# Import common functions.
> +. ./common/fuzzy
> +. ./common/filter
> +. ./common/dmerror
> +
> +# real QA test starts here
> +_supported_fs xfs
> +_require_dm_target error
> +_require_scratch
> +_require_scratch_xfs_crc
> +_require_scrub
> +
> +filter_scrub_errors() {
> +	_filter_scratch | sed \
> +		-e "s/offset $((fs_blksz * 2)) /offset 2FSB /g" \
> +		-e "s/length $fs_blksz.*/length 1FSB./g"
> +}
> +
> +_scratch_mkfs >> $seqres.full
> +_dmerror_init
> +_dmerror_mount >> $seqres.full 2>&1
> +
> +_supports_xfs_scrub $SCRATCH_MNT $SCRATCH_DEV || _notrun "Scrub not supported"
> +
> +# Write a file with 4 file blocks worth of data
> +victim=$SCRATCH_MNT/a
> +file_blksz=$(_get_file_block_size $SCRATCH_MNT)
> +$XFS_IO_PROG -f -c "pwrite -S 0x58 0 $((4 * file_blksz))" -c "fsync" $victim >> $seqres.full
> +unset errordev
> +_xfs_is_realtime_file $victim && errordev="RT"
> +bmap_str="$($XFS_IO_PROG -c "bmap -elpv" $victim | grep "^[[:space:]]*0:")"
> +echo "$errordev:$bmap_str" >> $seqres.full
> +
> +phys="$(echo "$bmap_str" | $AWK_PROG '{print $3}')"
> +if [ "$errordev" = "RT" ]; then
> +	len="$(echo "$bmap_str" | $AWK_PROG '{print $4}')"
> +else
> +	len="$(echo "$bmap_str" | $AWK_PROG '{print $6}')"
> +fi
> +fs_blksz=$(_get_block_size $SCRATCH_MNT)
> +echo "file_blksz:$file_blksz:fs_blksz:$fs_blksz" >> $seqres.full
> +kernel_sectors_per_fs_block=$((fs_blksz / 512))
> +
> +# Did we get at least 4 fs blocks worth of extent?
> +min_len_sectors=$(( 4 * kernel_sectors_per_fs_block ))
> +test "$len" -lt $min_len_sectors && \
> +	_fail "could not format a long enough extent on an empty fs??"
> +
> +phys_start=$(echo "$phys" | sed -e 's/\.\..*//g')
> +
> +echo "$errordev:$phys:$len:$fs_blksz:$phys_start" >> $seqres.full
> +echo "victim file:" >> $seqres.full
> +od -tx1 -Ad -c $victim >> $seqres.full
> +
> +# Set the dmerror table so that all IO will pass through.
> +_dmerror_reset_table
> +
> +cat >> $seqres.full << ENDL
> +dmerror before:
> +$DMERROR_TABLE
> +$DMERROR_RTTABLE
> +<end table>
> +ENDL
> +
> +# All sector numbers that we feed to the kernel must be in units of 512b, but
> +# they also must be aligned to the device's logical block size.
> +logical_block_size=$(_min_dio_alignment $SCRATCH_DEV)
> +kernel_sectors_per_device_lba=$((logical_block_size / 512))
> +
> +# Mark as bad one of the device LBAs in the middle of the extent.  Target the
> +# second LBA of the third block of the four-block file extent that we allocated
> +# earlier, but without overflowing into the fourth file block.
> +bad_sector=$(( phys_start + (2 * kernel_sectors_per_fs_block) ))
> +bad_len=$kernel_sectors_per_device_lba
> +if (( kernel_sectors_per_device_lba < kernel_sectors_per_fs_block )); then
> +	bad_sector=$((bad_sector + kernel_sectors_per_device_lba))
> +fi
> +if (( (bad_sector % kernel_sectors_per_device_lba) != 0)); then
> +	echo "bad_sector $bad_sector not congruent with device logical block size $logical_block_size"
> +fi
> +_dmerror_mark_range_bad $bad_sector $bad_len $errordev
> +
> +cat >> $seqres.full << ENDL
> +dmerror after marking bad:
> +$DMERROR_TABLE
> +$DMERROR_RTTABLE
> +<end table>
> +ENDL
> +
> +_dmerror_load_error_table
> +
> +# See if the media scan picks it up.
> +echo "Scrub for injected media error (single threaded)"
> +
> +# Once in single-threaded mode
> +_scratch_scrub -b -x >> $seqres.full 2> $tmp.error
> +cat $tmp.error | filter_scrub_errors
> +
> +# Once in parallel mode
> +echo "Scrub for injected media error (multi threaded)"
> +_scratch_scrub -x >> $seqres.full 2> $tmp.error
> +cat $tmp.error | filter_scrub_errors
> +
> +# Remount to flush the page cache and reread to see the IO error
> +_dmerror_unmount
> +_dmerror_mount
> +echo "victim file:" >> $seqres.full
> +od -tx1 -Ad -c $victim >> $seqres.full 2> $tmp.error
> +cat $tmp.error | sed -e 's/read error: //g' | _filter_scratch
> +
> +# Scrub again to re-confirm the media error across a remount
> +echo "Scrub for injected media error (after remount)"
> +_scratch_scrub -x >> $seqres.full 2> $tmp.error
> +cat $tmp.error | filter_scrub_errors
> +
> +# Now mark the bad range good so that a retest shows no media failure.
> +_dmerror_mark_range_good $bad_sector $bad_len $errordev
> +_dmerror_load_error_table
> +
> +cat >> $seqres.full << ENDL
> +dmerror after marking good:
> +$DMERROR_TABLE
> +$DMERROR_RTTABLE
> +<end table>
> +ENDL
> +
> +echo "Scrub after removing injected media error"
> +
> +# Scrub one last time to make sure the error's gone.
> +_scratch_scrub -x >> $seqres.full 2> $tmp.error
> +cat $tmp.error | filter_scrub_errors
> +
> +# success, all done
> +status=0
> +exit
> diff --git a/tests/xfs/747.out b/tests/xfs/747.out
> new file mode 100644
> index 0000000000..714ceb2e56
> --- /dev/null
> +++ b/tests/xfs/747.out
> @@ -0,0 +1,12 @@
> +QA output created by 747
> +Scrub for injected media error (single threaded)
> +Unfixable Error: SCRATCH_MNT/a: media error at data offset 2FSB length 1FSB.
> +SCRATCH_MNT: unfixable errors found: 1
> +Scrub for injected media error (multi threaded)
> +Unfixable Error: SCRATCH_MNT/a: media error at data offset 2FSB length 1FSB.
> +SCRATCH_MNT: unfixable errors found: 1
> +od: SCRATCH_MNT/a: Input/output error
> +Scrub for injected media error (after remount)
> +Unfixable Error: SCRATCH_MNT/a: media error at data offset 2FSB length 1FSB.
> +SCRATCH_MNT: unfixable errors found: 1
> +Scrub after removing injected media error
>
diff mbox series

Patch

diff --git a/common/dmerror b/common/dmerror
index 54122b12ea..3494b6dd3b 100644
--- a/common/dmerror
+++ b/common/dmerror
@@ -250,3 +250,150 @@  _dmerror_load_working_table()
 	[ $load_res -ne 0 ] && _fail "dmsetup failed to load error table"
 	[ $resume_res -ne 0 ] && _fail  "dmsetup resume failed"
 }
+
+# Given a list of (start, length) tuples on stdin, combine adjacent tuples into
+# larger ones and write the new list to stdout.
+__dmerror_combine_extents()
+{
+	local awk_program='
+	BEGIN {
+		start = 0; len = 0;
+	}
+	{
+		if (start + len == $1) {
+			len += $2;
+		} else {
+			if (len > 0)
+				printf("%d %d\n", start, len);
+			start = $1;
+			len = $2;
+		}
+	}
+	END {
+		if (len > 0)
+			printf("%d %d\n", start, len);
+	}'
+
+	awk "$awk_program"
+}
+
+# Given a block device, the name of a preferred dm target, the name of an
+# implied dm target, and a list of (start, len) tuples on stdin, create a new
+# dm table which maps each of the tuples to the preferred target and all other
+# areas to the implied dm target.
+__dmerror_recreate_map()
+{
+	local device="$1"
+	local preferred_tgt="$2"
+	local implied_tgt="$3"
+	local size=$(blockdev --getsz "$device")
+
+	local awk_program='
+	BEGIN {
+		implied_start = 0;
+	}
+	{
+		extent_start = $1;
+		extent_len = $2;
+
+		if (extent_start > size) {
+			extent_start = size;
+			extent_len = 0;
+		} else if (extent_start + extent_len > size) {
+			extent_len = size - extent_start;
+		}
+
+		if (implied_start < extent_start)
+			printf("%d %d %s %s %d\n", implied_start,
+					extent_start - implied_start,
+					implied_tgt, device, implied_start);
+		printf("%d %d %s %s %d\n", extent_start, extent_len,
+				preferred_tgt, device, extent_start);
+		implied_start = extent_start + extent_len;
+	}
+	END {
+		if (implied_start < size)
+			printf("%d %d %s %s %d\n", implied_start,
+					size - implied_start, implied_tgt,
+					device, implied_start);
+	}'
+
+	awk -v device="$device" -v size=$size -v implied_tgt="$implied_tgt" \
+		-v preferred_tgt="$preferred_tgt" "$awk_program"
+}
+
+# Update the dm error table so that the range (start, len) maps to the
+# preferred dm target, overriding anything that maps to the implied dm target.
+# This assumes that the only desired targets for this dm device are the
+# preferred and and implied targets.  The fifth argument is the scratch device
+# that we want to change the table for.
+__dmerror_change()
+{
+	local start="$1"
+	local len="$2"
+	local preferred_tgt="$3"
+	local implied_tgt="$4"
+	local whichdev="$5"
+	local old_table
+	local new_table
+
+	case "$whichdev" in
+	"SCRATCH_DEV"|"")	whichdev="$SCRATCH_DEV";;
+	"SCRATCH_LOGDEV"|"LOG")	whichdev="$NON_ERROR_LOGDEV";;
+	"SCRATCH_RTDEV"|"RT")	whichdev="$NON_ERROR_RTDEV";;
+	esac
+
+	case "$whichdev" in
+	"$SCRATCH_DEV")		old_table="$DMERROR_TABLE";;
+	"$NON_ERROR_LOGDEV")	old_table="$DMERROR_LOGTABLE";;
+	"$NON_ERROR_RTDEV")	old_table="$DMERROR_RTTABLE";;
+	*)
+		echo "$whichdev: Unknown dmerror device."
+		return
+		;;
+	esac
+
+	new_table="$( (echo "$old_table"; echo "$start $len $preferred_tgt") | \
+		awk -v type="$preferred_tgt" '{if ($3 == type) print $0;}' | \
+		sort -g | \
+		__dmerror_combine_extents | \
+		__dmerror_recreate_map "$whichdev" "$preferred_tgt" \
+				"$implied_tgt" )"
+
+	case "$whichdev" in
+	"$SCRATCH_DEV")		DMERROR_TABLE="$new_table";;
+	"$NON_ERROR_LOGDEV")	DMERROR_LOGTABLE="$new_table";;
+	"$NON_ERROR_RTDEV")	DMERROR_RTTABLE="$new_table";;
+	esac
+}
+
+# Reset the dm error table to everything ok.  The dm device itself must be
+# remapped by calling _dmerror_load_error_table.
+_dmerror_reset_table()
+{
+	DMERROR_TABLE="$DMLINEAR_TABLE"
+	DMERROR_LOGTABLE="$DMLINEAR_LOGTABLE"
+	DMERROR_RTTABLE="$DMLINEAR_RTTABLE"
+}
+
+# Update the dm error table so that IOs to the given range will return EIO.
+# The dm device itself must be remapped by calling _dmerror_load_error_table.
+_dmerror_mark_range_bad()
+{
+	local start="$1"
+	local len="$2"
+	local dev="$3"
+
+	__dmerror_change "$start" "$len" error linear "$dev"
+}
+
+# Update the dm error table so that IOs to the given range will succeed.
+# The dm device itself must be remapped by calling _dmerror_load_error_table.
+_dmerror_mark_range_good()
+{
+	local start="$1"
+	local len="$2"
+	local dev="$3"
+
+	__dmerror_change "$start" "$len" linear error "$dev"
+}
diff --git a/common/xfs b/common/xfs
index 8ac1964e9c..f466d2c42f 100644
--- a/common/xfs
+++ b/common/xfs
@@ -218,6 +218,15 @@  _xfs_get_dir_blocksize()
 	$XFS_INFO_PROG "$fs" | sed -n "s/^naming.*bsize=\([[:digit:]]*\).*/\1/p"
 }
 
+# Decide if this path is a file on the realtime device
+_xfs_is_realtime_file()
+{
+	if [ "$USE_EXTERNAL" != "yes" ] || [ -z "$SCRATCH_RTDEV" ]; then
+		return 1
+	fi
+	$XFS_IO_PROG -c 'stat -v' "$1" | grep -q -w realtime
+}
+
 # Set or clear the realtime status of every supplied path.  The first argument
 # is either 'data' or 'realtime'.  All other arguments should be paths to
 # existing directories or empty regular files.
diff --git a/tests/xfs/747 b/tests/xfs/747
new file mode 100755
index 0000000000..847e1e17f8
--- /dev/null
+++ b/tests/xfs/747
@@ -0,0 +1,155 @@ 
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0-or-later
+# Copyright (c) 2022 Oracle.  All Rights Reserved.
+#
+# FS QA Test No. 747
+#
+# Check xfs_scrub's media scan can actually return diagnostic information for
+# media errors in file data extents.
+
+. ./common/preamble
+_begin_fstest auto quick scrub eio
+
+# Override the default cleanup function.
+_cleanup()
+{
+	cd /
+	rm -f $tmp.*
+	_dmerror_cleanup
+}
+
+# Import common functions.
+. ./common/fuzzy
+. ./common/filter
+. ./common/dmerror
+
+# real QA test starts here
+_supported_fs xfs
+_require_dm_target error
+_require_scratch
+_require_scratch_xfs_crc
+_require_scrub
+
+filter_scrub_errors() {
+	_filter_scratch | sed \
+		-e "s/offset $((fs_blksz * 2)) /offset 2FSB /g" \
+		-e "s/length $fs_blksz.*/length 1FSB./g"
+}
+
+_scratch_mkfs >> $seqres.full
+_dmerror_init
+_dmerror_mount >> $seqres.full 2>&1
+
+_supports_xfs_scrub $SCRATCH_MNT $SCRATCH_DEV || _notrun "Scrub not supported"
+
+# Write a file with 4 file blocks worth of data
+victim=$SCRATCH_MNT/a
+file_blksz=$(_get_file_block_size $SCRATCH_MNT)
+$XFS_IO_PROG -f -c "pwrite -S 0x58 0 $((4 * file_blksz))" -c "fsync" $victim >> $seqres.full
+unset errordev
+_xfs_is_realtime_file $victim && errordev="RT"
+bmap_str="$($XFS_IO_PROG -c "bmap -elpv" $victim | grep "^[[:space:]]*0:")"
+echo "$errordev:$bmap_str" >> $seqres.full
+
+phys="$(echo "$bmap_str" | $AWK_PROG '{print $3}')"
+if [ "$errordev" = "RT" ]; then
+	len="$(echo "$bmap_str" | $AWK_PROG '{print $4}')"
+else
+	len="$(echo "$bmap_str" | $AWK_PROG '{print $6}')"
+fi
+fs_blksz=$(_get_block_size $SCRATCH_MNT)
+echo "file_blksz:$file_blksz:fs_blksz:$fs_blksz" >> $seqres.full
+kernel_sectors_per_fs_block=$((fs_blksz / 512))
+
+# Did we get at least 4 fs blocks worth of extent?
+min_len_sectors=$(( 4 * kernel_sectors_per_fs_block ))
+test "$len" -lt $min_len_sectors && \
+	_fail "could not format a long enough extent on an empty fs??"
+
+phys_start=$(echo "$phys" | sed -e 's/\.\..*//g')
+
+echo "$errordev:$phys:$len:$fs_blksz:$phys_start" >> $seqres.full
+echo "victim file:" >> $seqres.full
+od -tx1 -Ad -c $victim >> $seqres.full
+
+# Set the dmerror table so that all IO will pass through.
+_dmerror_reset_table
+
+cat >> $seqres.full << ENDL
+dmerror before:
+$DMERROR_TABLE
+$DMERROR_RTTABLE
+<end table>
+ENDL
+
+# All sector numbers that we feed to the kernel must be in units of 512b, but
+# they also must be aligned to the device's logical block size.
+logical_block_size=$(_min_dio_alignment $SCRATCH_DEV)
+kernel_sectors_per_device_lba=$((logical_block_size / 512))
+
+# Mark as bad one of the device LBAs in the middle of the extent.  Target the
+# second LBA of the third block of the four-block file extent that we allocated
+# earlier, but without overflowing into the fourth file block.
+bad_sector=$(( phys_start + (2 * kernel_sectors_per_fs_block) ))
+bad_len=$kernel_sectors_per_device_lba
+if (( kernel_sectors_per_device_lba < kernel_sectors_per_fs_block )); then
+	bad_sector=$((bad_sector + kernel_sectors_per_device_lba))
+fi
+if (( (bad_sector % kernel_sectors_per_device_lba) != 0)); then
+	echo "bad_sector $bad_sector not congruent with device logical block size $logical_block_size"
+fi
+_dmerror_mark_range_bad $bad_sector $bad_len $errordev
+
+cat >> $seqres.full << ENDL
+dmerror after marking bad:
+$DMERROR_TABLE
+$DMERROR_RTTABLE
+<end table>
+ENDL
+
+_dmerror_load_error_table
+
+# See if the media scan picks it up.
+echo "Scrub for injected media error (single threaded)"
+
+# Once in single-threaded mode
+_scratch_scrub -b -x >> $seqres.full 2> $tmp.error
+cat $tmp.error | filter_scrub_errors
+
+# Once in parallel mode
+echo "Scrub for injected media error (multi threaded)"
+_scratch_scrub -x >> $seqres.full 2> $tmp.error
+cat $tmp.error | filter_scrub_errors
+
+# Remount to flush the page cache and reread to see the IO error
+_dmerror_unmount
+_dmerror_mount
+echo "victim file:" >> $seqres.full
+od -tx1 -Ad -c $victim >> $seqres.full 2> $tmp.error
+cat $tmp.error | sed -e 's/read error: //g' | _filter_scratch
+
+# Scrub again to re-confirm the media error across a remount
+echo "Scrub for injected media error (after remount)"
+_scratch_scrub -x >> $seqres.full 2> $tmp.error
+cat $tmp.error | filter_scrub_errors
+
+# Now mark the bad range good so that a retest shows no media failure.
+_dmerror_mark_range_good $bad_sector $bad_len $errordev
+_dmerror_load_error_table
+
+cat >> $seqres.full << ENDL
+dmerror after marking good:
+$DMERROR_TABLE
+$DMERROR_RTTABLE
+<end table>
+ENDL
+
+echo "Scrub after removing injected media error"
+
+# Scrub one last time to make sure the error's gone.
+_scratch_scrub -x >> $seqres.full 2> $tmp.error
+cat $tmp.error | filter_scrub_errors
+
+# success, all done
+status=0
+exit
diff --git a/tests/xfs/747.out b/tests/xfs/747.out
new file mode 100644
index 0000000000..714ceb2e56
--- /dev/null
+++ b/tests/xfs/747.out
@@ -0,0 +1,12 @@ 
+QA output created by 747
+Scrub for injected media error (single threaded)
+Unfixable Error: SCRATCH_MNT/a: media error at data offset 2FSB length 1FSB.
+SCRATCH_MNT: unfixable errors found: 1
+Scrub for injected media error (multi threaded)
+Unfixable Error: SCRATCH_MNT/a: media error at data offset 2FSB length 1FSB.
+SCRATCH_MNT: unfixable errors found: 1
+od: SCRATCH_MNT/a: Input/output error
+Scrub for injected media error (after remount)
+Unfixable Error: SCRATCH_MNT/a: media error at data offset 2FSB length 1FSB.
+SCRATCH_MNT: unfixable errors found: 1
+Scrub after removing injected media error