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Message-ID: <aKbX-dBzSC1pmPuh@li-dc0c254c-257c-11b2-a85c-98b6c1322444.ibm.com>
Date: Thu, 21 Aug 2025 13:55:29 +0530
From: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To: John Garry <john.g.garry@...cle.com>
Cc: Zorro Lang <zlang@...hat.com>, fstests@...r.kernel.org,
Ritesh Harjani <ritesh.list@...il.com>, djwong@...nel.org,
tytso@....edu, linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-ext4@...r.kernel.org
Subject: Re: [PATCH v4 11/11] ext4: Atomic write test for extent split across
leaf nodes
On Wed, Aug 13, 2025 at 02:54:04PM +0100, John Garry wrote:
> 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@...ux.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.
Hey John thanks for the review. Sorry for the late reply i had a mini
vacation followed by lei suddenly not pulling emails :/
Anyways, yes I've added the $DEBUGFS command so we can observe the
extent structure, but the filefrag would look something like this (last
few extents):
...
337: 674.. 674: 10130.. 10130: 1:
338: 676.. 676: 10132.. 10132: 1:
339: 678.. 678: 10134.. 10134: 1:
340: 679.. 680: 10135.. 10136: 2: last,eof
Notice that the last 2 extents are logically and physically continuous
but not merged.
Reards,
ojaswin
>
> Thanks,
> John
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