Re: [PATCH v4 11/11] ext4: Atomic write test for extent split across leaf nodes

[John Garry <john.g.garry@oracle.com>]

On 10/08/2025 14:42, Ojaswin Mujoo wrote:
> 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     | 129 +++++++++++++++++++++++++++++++++++++++++++++
>   tests/ext4/063.out |   2 +
>   2 files changed, 131 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..40867acb
> --- /dev/null
> +++ b/tests/ext4/063
> @@ -0,0 +1,129 @@
> +#! /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. 

Nit: I assume that you mean "2 or more 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

tearing

> +# 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 with bs len extents at alternate offsets.
> +	# The tree should look as follows
> +	#
> +	#                    +---------+---------+
> +	#                    | index 1 | index 2 |
> +	#                    +-----+---+-----+---+
> +	#                   +------+         +-----------+
> +	#                   |                            |
> +	#      +-------+-------+---+---------+     +-----+----+
> +	#      | ex 1  | ex 2  |   |  ex n   |     |  ex n+1  |
> +	#      | off:0 | off:2 |...| off:678 |     |  off:680 |
> +	#      | len:1 | len:1 |   |  len:1  |     |   len:1  |
> +	#      +-------+-------+---+---------+     +----------+
> +	#
> +	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

don't

> +	# end up with a tree as:
> +	#
> +	#                    +---------+---------+
> +	#                    | index 1 | index 2 |
> +	#                    +-----+---+-----+---+
> +	#                   +------+         +------------+
> +	#                   |                             |
> +	#      +-------+-------+---+---------+     +------+-----------+
> +	#      | ex 1  | ex 2  |   |  ex n   |     |  ex n+1 (merged) |
> +	#      | off:0 | off:2 |...| off:678 |     |      off:679     |
> +	#      | len:1 | len:1 |   |  len:1  |     |      len:2       |
> +	#      +-------+-------+---+---------+     +------------------+
> +	#
> +	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
> +}
> +

Out of curiosity, for such a file with split extents, what would 
filefrag output look like? An example would be nice.

Thanks,
John