Re: [PATCH v3 12/13] ext4/063: Atomic write test for extent split across leaf nodes

Ojaswin Mujoo
2025-07-30T14:08:10Z

On Tue, Jul 29, 2025 at 12:41:54PM -0700, Darrick J. Wong wrote:
> On Sat, Jul 12, 2025 at 07:42:54PM +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@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
>
> Where did 684 come from? It's not in the 'before' diagram. Did
> "ex n + 1" previously map 682-684, and now it maps 681-684?

Okay so the 684 is a bit misleading as in there is nothing there.
The extent at 682 is len=1 and spans [682-683). Now that you pointed it
out, I think the 0..2...680 logicial offsets are confusing, since they
are actually ext4_extent.ee_block values but the diagram makes it seem
like they are indexes into the array of extents. Let me see if I can
make it better.

Thanks for the review!
ojaswin

>
> The rest looks ok though.
>
> --D
>