[PATCH fstests v3] generic: test Btrfs fsync vs. size-extending prealloc write crash

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From: Omar Sandoval <osandov@xxxxxx>

This is a regression test for a Btrfs bug, but there's nothing
Btrfs-specific about it. Since it's a race, we just try to make the race
happen in a loop and pass if it doesn't crash after all of our attempts.

Signed-off-by: Omar Sandoval <osandov@xxxxxx>
---
Changes from v2 [1]:

- Rebased on for-next.
- Added _require_odirect.
- Added FSTYP check to _fixed_by_kernel_commit.
- Added Filipe's Reviewed-by.

Changes from v1 [2]:

- Added missing groups and requires.
- Simplified $XFS_IO_PROG calls.
- Removed -i flag from $XFS_IO_PROG to make race reproduce more
  reliably.
- Removed all of the file creation and dump-tree parsing since the only
  file on a fresh filesystem is guaranteed to be at the end of a leaf
  anyways.
- Rewrote to be a generic test.

1: https://lore.kernel.org/linux-btrfs/d032e0b964f163229b684c0ac72b656ec9bf7b48.1716584019.git.osandov@xxxxxxxxxxx/
2: https://lore.kernel.org/linux-btrfs/297da2b53ce9b697d82d89afd322b2cc0d0f392d.1716492850.git.osandov@xxxxxxxxxxx/

 tests/generic/748     | 45 +++++++++++++++++++++++++++++++++++++++++++
 tests/generic/748.out |  2 ++
 2 files changed, 47 insertions(+)
 create mode 100755 tests/generic/748
 create mode 100644 tests/generic/748.out

diff --git a/tests/generic/748 b/tests/generic/748
new file mode 100755
index 00000000..2ec33694
--- /dev/null
+++ b/tests/generic/748
@@ -0,0 +1,45 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# FS QA Test 748
+#
+# Repeatedly prealloc beyond i_size, set an xattr, direct write into the
+# prealloc while extending i_size, then fdatasync. This is a regression test
+# for a Btrfs crash.
+#
+. ./common/preamble
+. ./common/attr
+_begin_fstest auto quick log preallocrw dangerous
+
+_supported_fs generic
+_require_scratch
+_require_attrs
+_require_odirect
+_require_xfs_io_command falloc -k
+[ "$FSTYP" = btrfs ] && _fixed_by_kernel_commit XXXXXXXXXXXX \
+	"btrfs: fix crash on racing fsync and size-extending write into prealloc"
+
+# -i slows down xfs_io startup and makes the race much less reliable.
+export XFS_IO_PROG="$(echo "$XFS_IO_PROG" | sed 's/ -i\b//')"
+
+_scratch_mkfs >> $seqres.full 2>&1 || _fail "mkfs failed"
+_scratch_mount
+
+blksz=$(_get_block_size "$SCRATCH_MNT")
+
+# On Btrfs, since this is the only file on the filesystem, its metadata is at
+# the end of a B-tree leaf. We want an ordered extent completion to add an
+# extent item at the end of the leaf while we're logging prealloc extents
+# beyond i_size after an xattr was set.
+for ((i = 0; i < 5000; i++)); do
+	$XFS_IO_PROG -ftd -c "falloc -k 0 $((blksz * 3))" -c "pwrite -q -w 0 $blksz" "$SCRATCH_MNT/file"
+	$SETFATTR_PROG -n user.a -v a "$SCRATCH_MNT/file"
+	$XFS_IO_PROG -d -c "pwrite -q -w $blksz $blksz" "$SCRATCH_MNT/file"
+done
+
+# If it didn't crash, we're good.
+
+echo "Silence is golden"
+status=0
+exit
diff --git a/tests/generic/748.out b/tests/generic/748.out
new file mode 100644
index 00000000..dc050a96
--- /dev/null
+++ b/tests/generic/748.out
@@ -0,0 +1,2 @@
+QA output created by 748
+Silence is golden
-- 
2.45.1





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