[PATCH v2 12/13] ext4/063: Atomic write test for extent split across leaf nodes

Ojaswin Mujoo
2025-06-26T11:59:40Z

In ext4, even if an allocated range is physically and logically
contiguous, it can still be split into 2 extents. This is because ext4
does not merge extents across leaf nodes. This is an issue for atomic
writes since even for a continuous extent the map block could (in rare
cases) return a shorter map, hence tearning the write. This test creates
such a file and ensures that the atomic write handles this case
correctly

Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
---
tests/ext4/063 | 125 +++++++++++++++++++++++++++++++++++++++++++++
tests/ext4/063.out | 2 +
2 files changed, 127 insertions(+)
create mode 100755 tests/ext4/063
create mode 100644 tests/ext4/063.out

diff --git a/tests/ext4/063 b/tests/ext4/063
new file mode 100755
index 00000000..25b5693d
--- /dev/null
+++ b/tests/ext4/063
@@ -0,0 +1,125 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2025 IBM Corporation. All Rights Reserved.
+#
+# In ext4, even if an allocated range is physically and logically contiguous,
+# it can still be split into 2 extents. This is because ext4 does not merge
+# extents across leaf nodes. This is an issue for atomic writes since even for
+# a continuous extent the map block could (in rare cases) return a shorter map,
+# hence tearning the write. This test creates such a file and ensures that the
+# atomic write handles this case correctly
+#
+. ./common/preamble
+. ./common/atomicwrites
+_begin_fstest auto atomicwrites
+
+_require_scratch_write_atomic_multi_fsblock
+_require_atomic_write_test_commands
+_require_command "$DEBUGFS_PROG" debugfs
+
+prep() {
+ local bs=`_get_block_size $SCRATCH_MNT`
+ local ex_hdr_bytes=12
+ local ex_entry_bytes=12
+ local entries_per_blk=$(( (bs - ex_hdr_bytes) / ex_entry_bytes ))
+
+ # fill the extent tree leaf which bs len extents at alternate offsets. For example,
+ # for 4k bs the tree should look as follows
+ #
+ # +---------+---------+
+ # | index 1 | index 2 |
+ # +-----+---+-----+---+
+ # +--------+ +-------+
+ # | |
+ # +----------+--------------+ +-----+-----+
+ # | ex 1 | ex 2 |... | ex n | | ex n + 1 |
+ # +-------------------------+ +-----------+
+ # 0 2 680 682
+ for i in $(seq 0 $entries_per_blk)
+ do
+ $XFS_IO_PROG -fc "pwrite -b $bs $((i * 2 * bs)) $bs" $testfile > /dev/null
+ done
+ sync $testfile
+
+ echo >> $seqres.full
+ echo "Create file with extents spanning 2 leaves. Extents:">> $seqres.full
+ echo "...">> $seqres.full
+ $DEBUGFS_PROG -R "ex `basename $testfile`" $SCRATCH_DEV |& tail >> $seqres.full
+
+ # Now try to insert a new extent ex(new) between ex(n) and ex(n+1). Since
+ # this is a new FS the allocator would find continuous blocks such that
+ # ex(n) ex(new) ex(n+1) are physically(and logically) contiguous. However,
+ # since we dont merge extents across leaf we will end up with a tree as:
+ #
+ # +---------+---------+
+ # | index 1 | index 2 |
+ # +-----+---+-----+---+
+ # +--------+ +-------+
+ # | |
+ # +----------+--------------+ +-----+-----+
+ # | ex 1 | ex 2 |... | ex n | | ex merged |
+ # +-------------------------+ +-----------+
+ # 0 2 680 681 682 684
+ #
+ echo >> $seqres.full
+ torn_ex_offset=$((((entries_per_blk * 2) - 1) * bs))
+ $XFS_IO_PROG -c "pwrite $torn_ex_offset $bs" $testfile >> /dev/null
+ sync $testfile
+
+ echo >> $seqres.full
+ echo "Perform 1 block write at $torn_ex_offset to create torn extent. Extents:">> $seqres.full
+ echo "...">> $seqres.full
+ $DEBUGFS_PROG -R "ex `basename $testfile`" $SCRATCH_DEV |& tail >> $seqres.full
+
+ _scratch_cycle_mount
+}
+
+_scratch_mkfs >> $seqres.full
+_scratch_mount >> $seqres.full
+
+testfile=$SCRATCH_MNT/testfile
+touch $testfile
+awu_max=$(_get_atomic_write_unit_max $testfile)
+
+echo >> $seqres.full
+echo "# Prepping the file" >> $seqres.full
+prep
+
+torn_aw_offset=$((torn_ex_offset - (torn_ex_offset % awu_max)))
+
+echo >> $seqres.full
+echo "# Performing atomic IO on the torn extent range. Command: " >> $seqres.full
+echo $XFS_IO_PROG -c "open -fsd $testfile" -c "pwrite -S 0x61 -DA -V1 -b $awu_max $torn_aw_offset $awu_max" >> $seqres.full
+$XFS_IO_PROG -c "open -fsd $testfile" -c "pwrite -S 0x61 -DA -V1 -b $awu_max $torn_aw_offset $awu_max" >> $seqres.full
+
+echo >> $seqres.full
+echo "Extent state after atomic write:">> $seqres.full
+echo "...">> $seqres.full
+$DEBUGFS_PROG -R "ex `basename $testfile`" $SCRATCH_DEV |& tail >> $seqres.full
+
+echo >> $seqres.full
+echo "# Checking data integrity" >> $seqres.full
+
+# create a dummy file with expected data
+$XFS_IO_PROG -fc "pwrite -S 0x61 -b $awu_max 0 $awu_max" $testfile.exp >> /dev/null
+expected_data=$(od -An -t x1 -j 0 -N $awu_max $testfile.exp)
+
+# We ensure that the data after atomic writes should match the expected data
+actual_data=$(od -An -t x1 -j $torn_aw_offset -N $awu_max $testfile)
+if [[ "$actual_data" != "$expected_data" ]]
+then
+ echo "Checksum match failed at off: $torn_aw_offset size: $awu_max"
+ echo
+ echo "Expected: "
+ echo "$expected_data"
+ echo
+ echo "Actual contents: "
+ echo "$actual_data"
+
+ _fail
+fi
+
+echo -n "Data verification at offset $torn_aw_offset suceeded!" >> $seqres.full
+echo "Silence is golden"
+status=0
+exit
diff --git a/tests/ext4/063.out b/tests/ext4/063.out
new file mode 100644
index 00000000..de35fc52
--- /dev/null
+++ b/tests/ext4/063.out
@@ -0,0 +1,2 @@
+QA output created by 063
+Silence is golden
--
2.49.0