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

[Omar Sandoval <osandov@osandov.com>]

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"

> (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?

> > +# 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$gap_i"
> > +done
> > +
> > +# Create enough free space records to make the bnobt and cntbt both full,
> > +# 2-level trees, plus one more record to make them split all the way to the
> > +# root and become 3-level trees. After this, there is a 7-block free extent in
> > +# the rightmost leaf of the cntbt, and all of the leaves of the cntbt other
> > +# than the rightmost two are full. Without the fix, the free list is also
> > +# empty.
> > +$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.

Thanks!

Omar