Re: [PATCH 1/1] xfs: check for COW overflows in i_delayed_blks

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Jun 06, 2019 at 03:14:50PM -0400, Brian Foster wrote:
> On Tue, Jun 04, 2019 at 02:17:10PM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > With the new copy on write functionality it's possible to reserve so
> > much COW space for a file that we end up overflowing i_delayed_blks.
> > The only user-visible effect of this is to cause totally wrong i_blocks
> > output in stat, so check for that.
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> >  tests/xfs/907     |  223 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  tests/xfs/907.out |    8 ++
> >  tests/xfs/group   |    1 
> >  3 files changed, 232 insertions(+)
> >  create mode 100755 tests/xfs/907
> >  create mode 100644 tests/xfs/907.out
> > 
> > 
> > diff --git a/tests/xfs/907 b/tests/xfs/907
> > new file mode 100755
> > index 00000000..d85f12da
> > --- /dev/null
> > +++ b/tests/xfs/907
> > @@ -0,0 +1,223 @@
> > +#! /bin/bash
> > +# SPDX-License-Identifier: GPL-2.0+
> > +# Copyright (c) 2019 Oracle, Inc.  All Rights Reserved.
> > +#
> > +# FS QA Test No. 907
> > +#
> > +# Try to overflow i_delayed_blks by setting the largest cowextsize hint
> > +# possible, creating a sparse file with a single byte every cowextsize bytes,
> > +# reflinking it, and retouching every written byte to see if we can create
> > +# enough speculative COW reservations to overflow i_delayed_blks.
> > +#
> > +seq=`basename $0`
> > +seqres=$RESULT_DIR/$seq
> > +echo "QA output created by $seq"
> > +
> > +here=`pwd`
> > +tmp=/tmp/$$
> > +status=1	# failure is the default!
> > +trap "_cleanup; exit \$status" 0 1 2 3 7 15
> > +
> > +_cleanup()
> > +{
> > +	cd /
> > +	test -n "$loop_mount" && $UMOUNT_PROG $loop_mount > /dev/null 2>&1
> > +	test -n "$loop_dev" && _destroy_loop_device $loop_dev
> > +	rm -rf $tmp.*
> > +}
> > +
> > +# get standard environment, filters and checks
> > +. ./common/rc
> > +. ./common/reflink
> > +. ./common/filter
> > +
> > +# real QA test starts here
> > +_supported_os Linux
> > +_supported_fs xfs
> > +_require_scratch_reflink
> > +_require_cp_reflink
> > +_require_loop
> > +_require_xfs_debug	# needed for xfs_bmap -c
> > +
> > +MAXEXTLEN=2097151	# cowextsize can't be more than MAXEXTLEN
> > +
> > +echo "Format and mount"
> > +_scratch_mkfs > "$seqres.full" 2>&1
> > +_scratch_mount
> > +
> > +# Create a huge sparse filesystem on the scratch device because that's what
> > +# we're going to need to guarantee that we have enough blocks to overflow in
> > +# the first place.  We need to have at least enough free space on that huge fs
> > +# to handle one written block every MAXEXTLEN blocks and to reserve 2^32 blocks
> > +# in the COW fork.  There needs to be sufficient space in the scratch
> > +# filesystem to handle a 256M log, all the per-AG metadata, and all the data
> > +# written to the test file.
> > +#
> > +# Worst case, a 64k-block fs needs to be about 300TB.  Best case, a 1k block
> > +# filesystem needs ~5TB.  For the most common 4k case we only need a ~20TB fs.
> > +#
> > +# nr_cows is the number of writes we make to the filesystem.
> > +# blks_needed is the number of free blocks we need in the fs to trigger the
> > +#     overflow.  The destination file needs to have more than 2^32 blocks
> > +#     reserved for COW, and the source file needs to have 1 block written every
> > +#     MAXEXTLEN blocks (i.e. 2^32/MAXEXTLEN blocks) to set up the destination
> > +#     file.
> > +# loop_file_sz is the size of the filesystem we have to create to produce the
> > +#     overflow problems.  This is 20% more than blks_needed, and rounded to the
> > +#     nearest 512b so losetup doesn't whine.
> > +# est_nr_ags is a guess at the AG count, assuming that we'll have one AG per
> > +#     terabyte (which assumes that we need a multi-terabyte filesystem for this
> > +#     test).  We assume we'll need 16 blocks for AG metadata and 128K for inode
> > +#     chunks.
> > +# space_reqd_kb is the size of the huge sparse filesystem --
> > +#     256M for the log, per-AG metadata, and 10% more space than however much
> > +#     we will write to both test files.
> > +blksz=$(_get_file_block_size "$SCRATCH_MNT")
> > +nr_cows="$(( ((2 ** 32) / MAXEXTLEN) + 100 ))"
> > +blks_needed="$(( nr_cows * (1 + MAXEXTLEN) ))"
> > +loop_file_sz="$(( ((blksz * blks_needed) * 12 / 10) / 512 * 512 ))"
> > +est_nr_ags="$(( (loop_file_sz / (2 ** 40)) + 1 ))"
> > +est_ag_space_kb="$(( (est_nr_ags * 16 * blksz / 1024) + 128 ))"
> > +est_file_space_kb="$(( (((nr_cows * blksz) * 11 / 10) / 1024) ))"
> > +space_reqd_kb="$(( 262144 + est_ag_space_kb + 2 * est_file_space_kb ))"
> > +cat >> $seqres.full << ENDL
> > +blksz: $blksz
> > +nr_cows: $nr_cows
> > +blks_needed: $blks_needed
> > +loop_file_sz: $loop_file_sz
> > +est_nr_ags: $est_nr_ags
> > +est_ag_space_kb: $est_ag_space_kb
> > +est_file_space_kb: $est_file_space_kb
> > +space_reqd_kb: $space_reqd_kb
> > +ENDL
> > +_require_fs_space $SCRATCH_MNT $space_reqd_kb
> > +
> 
> This seems like it would be simpler to just create the worst case sparse
> fs (say 300TB or so) then make sure the scratch device has enough free
> blocks to accommodate the number of COWs (plus slop) that need to occur
> in the loop fs to overflow the counter. I don't have a 64k box handy,
> but the metadata size difference between a sparse 20TB fs and 300TB fs
> over a 4k scratch fs is only a few MB. Hm?

<shrug> Originally it did just hardcode 300T, and Eryu complained that
xfs_db would OOM when it tried to deal with a 300T filesystem.  I
changed the test to avoid xfs_db, but then it occurred to me (running on
a slow arm64 box) that the mkfs and xfs_repair runtimes could be cut
down considerably if the loop fs was only as large as it needed to be.

> > +loop_file=$SCRATCH_MNT/a.img
> > +loop_mount=$SCRATCH_MNT/a
> > +$XFS_IO_PROG -f -c "truncate $loop_file_sz" $loop_file
> > +loop_dev=$(_create_loop_device $loop_file)
> > +
> > +# Now we have to create the source file.  The goal is to overflow a 32-bit
> > +# i_delayed_blks, which means that we have to create at least that many delayed
> > +# allocation block reservations.  Take advantage of the fact that a cowextsize
> > +# hint causes creation of large speculative delalloc reservations in the cow
> > +# fork to reduce the amount of work we have to do.
> > +#
> > +# The maximum cowextsize can only be set to MAXEXTLEN fs blocks on a filesystem
> > +# whose AGs each have more than MAXEXTLEN * 2 blocks.  This we can do easily
> > +# with a multi-terabyte filesystem, so start by setting up the hint.  Note that
> > +# the current fsxattr interface specifies its u32 cowextsize hint in units of
> > +# bytes and therefore can't handle MAXEXTLEN * blksz on most filesystems, so we
> > +# set it via mkfs because mkfs takes units of fs blocks, not bytes.
> > +
> > +_mkfs_dev -d cowextsize=$MAXEXTLEN -l size=256m $loop_dev >> $seqres.full
> > +mkdir $loop_mount
> > +mount $loop_dev $loop_mount
> > +
> > +echo "Create crazy huge file"
> > +huge_file="$loop_mount/a"
> > +touch "$huge_file"
> > +blksz=$(_get_file_block_size "$loop_mount")
> > +extsize_bytes="$(( MAXEXTLEN * blksz ))"
> > +
> > +# Make sure it actually set a hint.
> > +curr_cowextsize_str="$($XFS_IO_PROG -c 'cowextsize' "$huge_file")"
> > +echo "$curr_cowextsize_str" >> $seqres.full
> > +cowextsize_bytes="$(echo "$curr_cowextsize_str" | sed -e 's/^.\([0-9]*\).*$/\1/g')"
> > +test "$cowextsize_bytes" -eq 0 && echo "could not set cowextsize?"
> > +
> > +# Now we have to seed the file with sparse contents.  Remember, the goal is to
> > +# create a little more than 2^32 delayed allocation blocks in the COW fork with
> > +# as little effort as possible.  We know that speculative COW preallocation
> > +# will create MAXEXTLEN-length reservations for us, so that means we should
> > +# be able to get away with touching a single byte every extsize_bytes.  We
> > +# do this backwards to avoid having to move EOF.
> > +seq $nr_cows -1 0 | while read n; do
> > +	off="$((n * extsize_bytes))"
> > +	$XFS_IO_PROG -c "pwrite $off 1" "$huge_file" > /dev/null
> > +done
> > +
> > +echo "Reflink crazy huge file"
> > +_cp_reflink "$huge_file" "$huge_file.b"
> > +
> > +# Now that we've shared all the blocks in the file, we touch them all again
> > +# to create speculative COW preallocations.
> > +echo "COW crazy huge file"
> > +seq $nr_cows -1 0 | while read n; do
> > +	off="$((n * extsize_bytes))"
> > +	$XFS_IO_PROG -c "pwrite $off 1" "$huge_file" > /dev/null
> > +done
> > +
> > +# Compare the number of blocks allocated to this file (as reported by stat)
> > +# against the number of blocks that are in the COW fork.  If either one is
> > +# less than 2^32 then we have evidence of an overflow problem.
> > +echo "Check crazy huge file"
> > +allocated_stat_blocks="$(stat -c %b "$huge_file")"
> > +stat_blksz="$(stat -c %B "$huge_file")"
> > +allocated_fsblocks=$(( allocated_stat_blocks * stat_blksz / blksz ))
> > +
> > +# Make sure we got enough COW reservations to overflow a 32-bit counter.
> > +
> > +# Return the number of delalloc & real blocks given bmap output for a fork of a
> > +# file.  Output is in units of 512-byte blocks.
> > +count_fork_blocks() {
> > +	$AWK_PROG "
> > +{
> > +	if (\$3 == \"delalloc\") {
> > +		x += \$4;
> > +	} else if (\$3 == \"hole\") {
> > +		;
> > +	} else {
> > +		x += \$6;
> > +	}
> > +}
> > +END {
> > +	print(x);
> > +}
> > +"
> > +}
> > +
> > +# Count the number of blocks allocated to a file based on the xfs_bmap output.
> > +# Output is in units of filesystem blocks.
> > +count_file_fork_blocks() {
> > +	local tag="$1"
> > +	local file="$2"
> > +	local args="$3"
> > +
> > +	$XFS_IO_PROG -c "bmap $args -l -p -v" "$huge_file" > $tmp.extents
> > +	echo "$tag fork map" >> $seqres.full
> > +	cat $tmp.extents >> $seqres.full
> > +	local sectors="$(count_fork_blocks < $tmp.extents)"
> > +	echo "$(( sectors / (blksz / 512) ))"
> > +}
> > +
> > +cowblocks=$(count_file_fork_blocks cow "$huge_file" "-c")
> > +attrblocks=$(count_file_fork_blocks attr "$huge_file" "-a")
> > +datablocks=$(count_file_fork_blocks data "$huge_file" "")
> > +
> > +# Did we create more than 2^32 blocks in the cow fork?
> > +echo "datablocks is $datablocks" >> $seqres.full
> > +echo "attrblocks is $attrblocks" >> $seqres.full
> > +echo "cowblocks is $cowblocks" >> $seqres.full
> > +test "$cowblocks" -lt $((2 ** 32)) && \
> > +	echo "cowblocks (${cowblocks}) should be more than 2^32!"

This part checks that the test did what we expect.

> > +
> > +# Does stat's block allocation count exceed 2^32?
> > +echo "stat blocks is $allocated_fsblocks" >> $seqres.full
> > +test "$allocated_fsblocks" -lt $((2 ** 32)) && \
> > +	echo "stat blocks (${allocated_fsblocks}) should be more than 2^32!"

This part detects the problem in the incore state.

> > +# Finally, does st_blocks match what we computed from the forks?
> > +expected_allocated_fsblocks=$((datablocks + cowblocks + attrblocks))
> > +echo "expected stat blocks is $expected_allocated_fsblocks" >> $seqres.full
> > +
> > +_within_tolerance "st_blocks" $allocated_fsblocks $expected_allocated_fsblocks 2% -v

And this one on is a second sanity check that the test did roughly what
we expected.

> > +
> 
> Similar question with the post-processing stuff... how much of this is
> to detect the problem vs. determine the test sequence did what we
> expect?

(Answered above.)

> Does the fs check below report corruption or is it purely
> in-core state that ends up broken?

It's only the incore state that ends up broken.

--D

> Brian
> 
> > +echo "Test done"
> > +# Quick check the large sparse fs, but skip xfs_db because it doesn't scale
> > +# well on a multi-terabyte filesystem.
> > +LARGE_SCRATCH_DEV=yes _check_xfs_filesystem $loop_dev none none
> > +
> > +# success, all done
> > +status=0
> > +exit
> > diff --git a/tests/xfs/907.out b/tests/xfs/907.out
> > new file mode 100644
> > index 00000000..cc07d659
> > --- /dev/null
> > +++ b/tests/xfs/907.out
> > @@ -0,0 +1,8 @@
> > +QA output created by 907
> > +Format and mount
> > +Create crazy huge file
> > +Reflink crazy huge file
> > +COW crazy huge file
> > +Check crazy huge file
> > +st_blocks is in range
> > +Test done
> > diff --git a/tests/xfs/group b/tests/xfs/group
> > index ffe4ae12..e528c559 100644
> > --- a/tests/xfs/group
> > +++ b/tests/xfs/group
> > @@ -504,3 +504,4 @@
> >  504 auto quick mkfs label
> >  505 auto quick spaceman
> >  506 auto quick health
> > +907 clone
> >