From: Darrick J. Wong <djwong@xxxxxxxxxx>
This is a regression test for a data corruption bug that existed in XFS'
copy on write code between 4.9 and 4.19. The root cause is a
concurrency bug wherein we would drop ILOCK_SHARED after querying the
CoW fork in xfs_map_cow and retake it before querying the data fork in
xfs_map_blocks. See the test description for a lot more details.
Cc: Wengang Wang <wen.gang.wang@xxxxxxxxxx>
Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>
---
common/rc | 15 ++++
common/tracing | 66 +++++++++++++++++
tests/xfs/924 | 210 +++++++++++++++++++++++++++++++++++++++++++++++++++++
tests/xfs/924.out | 2 +
4 files changed, 293 insertions(+)
create mode 100644 common/tracing
create mode 100755 tests/xfs/924
create mode 100644 tests/xfs/924.out
diff --git a/common/rc b/common/rc
index 67bd74dc89..cac207e6b4 100644
--- a/common/rc
+++ b/common/rc
@@ -3604,6 +3604,21 @@ _check_xflag()
fi
}
+# Make sure the given file access mode is set to use the pagecache. If
+# userspace or kernel don't support statx or STATX_ATTR_DAX, we assume that
+# means pagecache. The sole parameter must be a directory.
+_require_pagecache_access() {
+ local testfile="$1/testfile"
+
+ touch "$testfile"
+ if ! _check_s_dax "$testfile" 0 &>> $seqres.full; then
+ rm -f "$testfile"
+ _notrun 'test requires pagecache access'
+ fi
+
+ rm -f "$testfile"
+}
+
# Check if dax mount options are supported
#
# $1 can be either 'dax=always' or 'dax'
diff --git a/common/tracing b/common/tracing
new file mode 100644
index 0000000000..b3051c27a7
--- /dev/null
+++ b/common/tracing
@@ -0,0 +1,66 @@
+##/bin/bash
+# SPDX-License-Identifier: GPL-2.0-or-later
+# Copyright (c) 2022 Oracle. All Rights Reserved.
+#
+# Routines for dealing with ftrace (or any other tracing).
+
+FTRACE_INSTANCES_DIR="/sys/kernel/debug/tracing/instances/"
+
+_require_ftrace() {
+ test -d "$FTRACE_INSTANCES_DIR" || \
+ _notrun "kernel does not support ftrace"
+}
+
+_ftrace_cleanup() {
+ if [ -d "$FTRACE_DIR" ]; then
+ _ftrace_ignore_events
+ # Removing an ftrace buffer requires rmdir, even though the
+ # virtual directory contains children.
+ rmdir "$FTRACE_DIR"
+ fi
+}
+
+_ftrace_setup() {
+ test -n "$FTRACE_DIR" && _fail "_ftrace_setup already run?"
+
+ # Give this fstest its own ftrace buffer so that we don't mess up
+ # any other tracers that might be running.
+ FTRACE_DIR="$FTRACE_INSTANCES_DIR/fstests.$seq"
+
+ test -d "$FTRACE_DIR" && rmdir "$FTRACE_DIR"
+ mkdir "$FTRACE_DIR"
+}
+
+# Intercept the given events. Arguments may be regular expressions.
+_ftrace_record_events() {
+ test -d "$FTRACE_DIR" || _fail "_ftrace_setup not run?"
+
+ for arg in "$@"; do
+ # Replace slashes with semicolons per ftrace convention
+ find "$FTRACE_DIR/events/" -type d -name "$arg" -printf '%P\n' | \
+ tr '/' ':' >> "$FTRACE_DIR/set_event"
+ done
+}
+
+# Stop intercepting the given events. If no arguments, stops all events.
+_ftrace_ignore_events() {
+ test -d "$FTRACE_DIR" || _fail "_ftrace_setup not run?"
+
+ if [ "$#" -eq 0 ]; then
+ echo > "$FTRACE_DIR/set_event"
+ else
+ for arg in "$@"; do
+ # Replace slashes with semicolons per ftrace convention
+ find "$FTRACE_DIR/events/" -type d -name "$arg" -printf '!%P\n' | \
+ tr '/' ':' >> "$FTRACE_DIR/set_event"
+ done
+ fi
+}
+
+# Dump whatever was written to the ftrace buffer since the last time this
+# helper was called.
+_ftrace_dump() {
+ test -d "$FTRACE_DIR" || _fail "_ftrace_setup not run?"
+
+ cat "$FTRACE_DIR/trace"
+}
diff --git a/tests/xfs/924 b/tests/xfs/924
new file mode 100755
index 0000000000..ce566c9259
--- /dev/null
+++ b/tests/xfs/924
@@ -0,0 +1,210 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2022 Oracle. All Rights Reserved.
+#
+# FS QA Test 924
+#
+# This is a regression test for a data corruption bug that existed in XFS' copy
+# on write code between 4.9 and 4.19. The root cause is a concurrency bug
+# wherein we would drop ILOCK_SHARED after querying the CoW fork in xfs_map_cow
+# and retake it before querying the data fork in xfs_map_blocks. If a second
+# thread changes the CoW fork mappings between the two calls, it's possible for
+# xfs_map_blocks to return a zero-block mapping, which results in writeback
+# being elided for that block. Elided writeback of dirty data results in
+# silent loss of writes.
+#
+# Worse yet, kernels from that era still used buffer heads, which means that an
+# elided writeback leaves the page clean but the bufferheads dirty. Due to a
+# naïve optimization in mark_buffer_dirty, the SetPageDirty call is elided if
+# the bufferhead is dirty, which means that a subsequent rewrite of the data
+# block will never result in the page being marked dirty, and all subsequent
+# writes are lost.
+#
+# It turns out that Christoph Hellwig unwittingly fixed the race in commit
+# 5c665e5b5af6 ("xfs: remove xfs_map_cow"), and no testcase was ever written.
+# Four years later, we hit it on a production 4.14 kernel. This testcase
+# relies on a debugging knob that introduces artificial delays into writeback.
+#
+# Before the race, the file blocks 0-1 are not shared and blocks 2-5 are
+# shared. There are no extents in CoW fork.
+#
+# Two threads race like this:
+#
+# Thread 1 (writeback block 0) | Thread 2 (write to block 2)
+# ---------------------------------|--------------------------------
+# |
+# 1. Check if block 0 in CoW fork |
+# from xfs_map_cow. |
+# |
+# 2. Block 0 not found in CoW |
+# fork; the block is considered |
+# not shared. |
+# |
+# 3. xfs_map_blocks looks up data |
+# fork to get a map covering |
+# block 0. |
+# |
+# 4. It gets a data fork mapping |
+# for block 0 with length 2. |
+# |
+# | 1. A buffered write to block 2 sees
+# | that it is a shared block and no
+# | extent covers block 2 in CoW fork.
+# |
+# | It creates a new CoW fork mapping.
+# | Due to the cowextsize, the new
+# | extent starts at block 0 with
+# | length 128.
+# |
+# |
+# 5. It lookup CoW fork again to |
+# trim the map (0, 2) to a |
+# shared block boundary. |
+# |
+# 5a. It finds (0, 128) in CoW fork|
+# 5b. It trims the data fork map |
+# from (0, 1) to (0, 0) (!!!) |
+# |
+# 6. The xfs_imap_valid call after |
+# the xfs_map_blocks call checks|
+# if the mapping (0, 0) covers |
+# block 0. The result is "NO". |
+# |
+# 7. Since block 0 has no physical |
+# block mapped, it's not added |
+# to the ioend. This is the |
+# first problem. |
+# |
+# 8. xfs_add_to_ioend usually |
+# clears the bufferhead dirty |
+# flag Because this is skipped,|
+# we leave the page clean with |
+# the associated buffer head(s) |
+# dirty (the second problem). |
+# Now the dirty state is |
+# inconsistent.
+#
+# On newer kernels, this is also a functionality test for the ifork sequence
+# counter because the writeback completions will change the data fork and force
+# revalidations of the wb mapping.
+#
+. ./common/preamble
+_begin_fstest auto quick clone
+
+# Import common functions.
+. ./common/reflink
+. ./common/inject
+. ./common/tracing
+
+# real QA test starts here
+_cleanup()
+{
+ test -n "$sentryfile" && rm -f $sentryfile
+ wait
+ _ftrace_cleanup
+ cd /
+ rm -r -f $tmp.* $sentryfile $tracefile
+}
+
+# Modify as appropriate.
+_supported_fs xfs
+_fixed_by_kernel_commit 5c665e5b5af6 "xfs: remove xfs_map_cow"
+_require_ftrace
+_require_xfs_io_error_injection "wb_delay_ms"
+_require_scratch_reflink
+_require_cp_reflink
+
+_scratch_mkfs >> $seqres.full
+_scratch_mount >> $seqres.full
+
+# This is a pagecache test, so try to disable fsdax mode.
+$XFS_IO_PROG -c 'chattr -x' $SCRATCH_MNT &> $seqres.full
+_require_pagecache_access $SCRATCH_MNT
+
+blksz=65536
+_require_congruent_file_oplen $SCRATCH_MNT $blksz
+
+# Make sure we have sufficient extent size to create speculative CoW
+# preallocations.
+$XFS_IO_PROG -c 'cowextsize 1m' $SCRATCH_MNT
+
+# Write out a file with the first two blocks unshared and the rest shared.
+_pwrite_byte 0x59 0 $((160 * blksz)) $SCRATCH_MNT/file >> $seqres.full
+_pwrite_byte 0x59 0 $((160 * blksz)) $SCRATCH_MNT/file.compare >> $seqres.full
+sync
+
+_cp_reflink $SCRATCH_MNT/file $SCRATCH_MNT/file.reflink
+
+_pwrite_byte 0x58 0 $((2 * blksz)) $SCRATCH_MNT/file >> $seqres.full
+_pwrite_byte 0x58 0 $((2 * blksz)) $SCRATCH_MNT/file.compare >> $seqres.full
+sync
+
+# Avoid creation of large folios on newer kernels by cycling the mount and
+# immediately writing to the page cache.
+_scratch_cycle_mount
+
+# Write the same data to file.compare as we're about to do to file. Do this
+# before slowing down writeback to avoid unnecessary delay.
+_pwrite_byte 0x57 0 $((2 * blksz)) $SCRATCH_MNT/file.compare >> $seqres.full
+_pwrite_byte 0x56 $((2 * blksz)) $((2 * blksz)) $SCRATCH_MNT/file.compare >> $seqres.full
+sync
+
+# Introduce a half-second wait to each writeback block mapping call. This
+# gives us a chance to race speculative cow prealloc with writeback.
+_scratch_inject_error "wb_delay_ms" 500
+
+_ftrace_setup
+_ftrace_record_events 'xfs_wb*iomap_invalid'
+
+# Start thread 1 + writeback above
+$XFS_IO_PROG -c "pwrite -S 0x57 0 $((2 * blksz))" \
+ -c 'fsync' $SCRATCH_MNT/file >> $seqres.full &
+sleep 1
+
+# Start a sentry to look for evidence of invalidation tracepoint tripping. If
+# we see that, we know we've forced writeback to revalidate a mapping. The
+# test has been successful, so turn off the delay.
+sentryfile=$TEST_DIR/$seq.sentry
+tracefile=$TEST_DIR/$seq.ftrace
+wait_for_errortag() {
+ while [ -e "$sentryfile" ]; do
+ _ftrace_dump | grep iomap_invalid >> "$tracefile"
+ if grep -q iomap_invalid "$tracefile"; then
+ _scratch_inject_error "wb_delay_ms" 0
+ _ftrace_ignore_events
+ break;
+ fi
+ sleep 0.5
+ done
+}
+touch $sentryfile
+wait_for_errortag &
+
+# Start thread 2 to create the cowextsize reservation
+$XFS_IO_PROG -c "pwrite -S 0x56 $((2 * blksz)) $((2 * blksz))" \
+ -c 'fsync' $SCRATCH_MNT/file >> $seqres.full
+rm -f $sentryfile
+
+cat "$tracefile" >> $seqres.full
+grep -q iomap_invalid "$tracefile"
+saw_invalidation=$?
+
+# Flush everything to disk. If the bug manifests, then after the cycle,
+# file should have stale 0x58 in block 0 because we silently dropped a write.
+_scratch_cycle_mount
+
+if ! cmp -s $SCRATCH_MNT/file $SCRATCH_MNT/file.compare; then
+ echo file and file.compare do not match
+ $XFS_IO_PROG -c 'bmap -celpv' -c 'bmap -elpv' $SCRATCH_MNT/file &>> $seqres.full
+ echo file.compare
+ od -tx1 -Ad -c $SCRATCH_MNT/file.compare
+ echo file
+ od -tx1 -Ad -c $SCRATCH_MNT/file
+elif [ $saw_invalidation -ne 0 ]; then
+ # The files matched, but nothing got logged about the revalidation?
+ echo "Expected to hear about writeback iomap invalidations?"
+fi
+
+echo Silence is golden
+status=0
+exit
diff --git a/tests/xfs/924.out b/tests/xfs/924.out
new file mode 100644
index 0000000000..c6655da35a
--- /dev/null
+++ b/tests/xfs/924.out
@@ -0,0 +1,2 @@
+QA output created by 924
+Silence is golden
