Re: [PATCH fstests] xfs: test refilling AGFL after lots of btree splits

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Oct 25, 2023 at 01:15:43PM -0700, Omar Sandoval wrote:
> On Wed, Oct 25, 2023 at 08:27:02AM -0700, Darrick J. Wong wrote:
> > On Tue, Oct 24, 2023 at 04:37:42PM -0700, Omar Sandoval wrote:
> > > This is a regression test for patch "xfs: fix internal error from AGFL
> > > exhaustion"), which is not yet merged. Without the fix, it will fail
> > > with a "Structure needs cleaning" error.
> > 
> > Will look at the actual code patch next...
> > 
> > > Signed-off-by: Omar Sandoval <osandov@osandov.com>
> > > ---
> > >  tests/xfs/601     | 62 +++++++++++++++++++++++++++++++++++++++++++++++
> > >  tests/xfs/601.out |  2 ++
> > >  2 files changed, 64 insertions(+)
> > >  create mode 100755 tests/xfs/601
> > >  create mode 100644 tests/xfs/601.out
> > > 
> > > diff --git a/tests/xfs/601 b/tests/xfs/601
> > > new file mode 100755
> > > index 00000000..bbc5b443
> > > --- /dev/null
> > > +++ b/tests/xfs/601
> > > @@ -0,0 +1,62 @@
> > > +#! /bin/bash
> > > +# SPDX-License-Identifier: GPL-2.0
> > > +# Copyright (c) Meta Platforms, Inc. and affiliates.
> > > +#
> > > +# FS QA Test 601
> > > +#
> > > +# Regression test for patch "xfs: fix internal error from AGFL exhaustion".
> > > +#
> > > +. ./common/preamble
> > > +_begin_fstest auto prealloc punch
> > > +
> > > +. ./common/filter
> > > +
> > > +_supported_fs xfs
> > > +_require_scratch
> > > +_require_test_program punch-alternating
> > > +_fixed_by_kernel_commit XXXXXXXXXXXX "xfs: fix internal error from AGFL exhaustion"
> > > +
> > > +_scratch_mkfs -m rmapbt=0 | _filter_mkfs > /dev/null 2> "$tmp.mkfs"
> > 
> > Need to probe if mkfs.xfs actually supports rmapbt options first, since
> > this bug applies to old fses from before rmap even existed, right?
> 
> Good point. Something like:
> 
> opts=
> if $MKFS_XFS_PROG |& grep rmapbt; then
> 	opts="-m rmapbt=0"
> fi
> _scratch_mkfs $opts | _filter_mkfs > /dev/null 2> "$tmp.mkfs"

Yep, that works.

> > (Or: What changes are needed to make the reproducer work with rmapbt
> > enabled?)
> 
> We'd need to craft the filesystem in a way that a single operation
> splits and adds a new level to the bnobt, cntbt, and rmapbt all at the
> same time. It can probably be done, but I suspect it'd be much more
> complicated :(
> 
> > > +. "$tmp.mkfs"
> > > +_scratch_mount
> > > +
> > > +alloc_block_len=$((_fs_has_crcs ? 56 : 16))
> > > +allocbt_leaf_maxrecs=$(((dbsize - alloc_block_len) / 8))
> > > +allocbt_node_maxrecs=$(((dbsize - alloc_block_len) / 12))
> > > +
> > > +# Create a big file with a size such that the punches below create the exact
> > > +# free extents we want.
> > > +num_holes=$((allocbt_leaf_maxrecs * allocbt_node_maxrecs - 1))
> > > +$XFS_IO_PROG -c "falloc 0 $((9 * dbsize + num_holes * dbsize * 2))" -f "$SCRATCH_MNT/big"
> > 
> > What happens if the allocations are all in some other AG?  The scratch
> > device could be 100TB.
> 
> Yeah, this relies on all of the allocations going to AG 0, and the big
> fallocate getting one contiguous extent. That always happened for me on
> a few different sized filesystems, but I understand it's not guaranteed.
> Maybe I should create the filesystem with -d agcount=1?

Hmm.  xfs_repair is likely to get cranky about single-AG filesystems...

> > > +# Fill in any small free extents in AG 0. After this, there should be only one,
> > > +# large free extent.
> > > +_scratch_unmount
> > > +mapfile -t gaps < <($XFS_DB_PROG -c 'agf 0' -c 'addr cntroot' -c 'p recs' "$SCRATCH_DEV" |
> > > +	$SED_PROG -rn 's/^[0-9]+:\[[0-9]+,([0-9]+)\].*/\1/p' |
> > > +	tac | tail -n +2)
> > 
> > _scratch_xfs_db -c 'agf 0' -c 'addr cntroot' -c 'btdump' ?
> 
> Will fix.

> > > +_scratch_mount
> > > +for gap_i in "${!gaps[@]}"; do
> > > +	gap=${gaps[$gap_i]}
> > > +	$XFS_IO_PROG -c "falloc 0 $((gap * dbsize))" -f "$SCRATCH_MNT/gap$gap_i"
> > > +done

...but you could check that the AG 0 cntbt actually has one large free
extent, as the comment says should be the case.

> > > +
> > > +# Create enough free space records to make the bnobt and cntbt both full,
> > > +# 2-level trees, plus one more record to make them split all the way to the
> > > +# root and become 3-level trees. After this, there is a 7-block free extent in
> > > +# the rightmost leaf of the cntbt, and all of the leaves of the cntbt other
> > > +# than the rightmost two are full. Without the fix, the free list is also
> > > +# empty.
> > > +$XFS_IO_PROG -c "fpunch $dbsize $((7 * dbsize))" "$SCRATCH_MNT/big"
> > > +"$here/src/punch-alternating" -o 9 "$SCRATCH_MNT/big"
> > > +
> > > +# Do an arbitrary operation that refills the free list. Without the fix, this
> > > +# will allocate 6 blocks from the 7-block free extent in the rightmost leaf of
> > > +# the cntbt, then try to insert the remaining 1 block free extent in the
> > > +# leftmost leaf of the cntbt. But that leaf is full, so this tries to split the
> > > +# leaf and fails because the free list is empty.
> > > +$XFS_IO_PROG -c "fpunch 0 $dbsize" "$SCRATCH_MNT/big"
> > > +
> > > +echo "Silence is golden"
> > 
> > Without the fix applied, what happens now?  Does fpunch fail with EIO
> > to taint the golden output?
> 
> It fails with EFSCORRUPTED/EUCLEAN and prints an error message as noted
> in my commit message, yeah.

Cool!  Looking forward to the next revision. :)

--D

> 
> Thanks!
> 
> Omar