On Tue, Aug 12, 2025 at 10:19:35AM -0700, Darrick J. Wong wrote:
> On Sun, Aug 10, 2025 at 07:12:02PM +0530, 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@xxxxxxxxxxxxx>
> > ---
> > 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. 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 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
> > + # 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 |
> > + # +-------+-------+---+---------+ +------------------+
> > + #
>
> Thanks for the nice picture demonstrating what you're trying to test!
> Reviewed-by: "Darrick J. Wong" <djwong@xxxxxxxxxx>
Sure, thanks for the suggestions and review!
Regards,
ojaswin
