Still reproducible on 4.20.7. The behavior is slightly different on current kernels (4.20.7, 4.14.96) which makes the problem a bit more difficult to detect.
# repro-hole-corruption-test
i: 91, status: 0, bytes_deduped: 131072
i: 92, status: 0, bytes_deduped: 131072
i: 93, status: 0, bytes_deduped: 131072
i: 94, status: 0, bytes_deduped: 131072
i: 95, status: 0, bytes_deduped: 131072
i: 96, status: 0, bytes_deduped: 131072
i: 97, status: 0, bytes_deduped: 131072
i: 98, status: 0, bytes_deduped: 131072
i: 99, status: 0, bytes_deduped: 131072
13107200 total bytes deduped in this operation
am: 4.8 MiB (4964352 bytes) converted to sparse holes.
94a8acd3e1f6e14272f3262a8aa73ab6b25c9ce8 am
6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
The sha1sum seems stable after the first drop_caches--until a second
process tries to read the test file:
6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
# cat am > /dev/null (in another shell)
19294e695272c42edb89ceee24bb08c13473140a am
6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
On Wed, Aug 22, 2018 at 11:11:25PM -0400, Zygo Blaxell wrote:
> This is a repro script for a btrfs bug that causes corrupted data reads
> when reading a mix of compressed extents and holes. The bug is
> reproducible on at least kernels v4.1..v4.18.
>
> Some more observations and background follow, but first here is the
> script and some sample output:
>
> root@rescue:/test# cat repro-hole-corruption-test
> #!/bin/bash
>
> # Write a 4096 byte block of something
> block () { head -c 4096 /dev/zero | tr '\0' "\\$1"; }
>
> # Here is some test data with holes in it:
> for y in $(seq 0 100); do
> for x in 0 1; do
> block 0;
> block 21;
> block 0;
> block 22;
> block 0;
> block 0;
> block 43;
> block 44;
> block 0;
> block 0;
> block 61;
> block 62;
> block 63;
> block 64;
> block 65;
> block 66;
> done
> done > am
> sync
>
> # Now replace those 101 distinct extents with 101 references to the
> first extent
> btrfs-extent-same 131072 $(for x in $(seq 0 100); do echo am $((x *
> 131072)); done) 2>&1 | tail
>
> # Punch holes into the extent refs
> fallocate -v -d am
>
> # Do some other stuff on the machine while this runs, and watch the
> sha1sums change!
> while :; do echo $(sha1sum am); sysctl -q vm.drop_caches={1,2,3}; sleep
> 1; done
>
> root@rescue:/test# ./repro-hole-corruption-test
> i: 91, status: 0, bytes_deduped: 131072
> i: 92, status: 0, bytes_deduped: 131072
> i: 93, status: 0, bytes_deduped: 131072
> i: 94, status: 0, bytes_deduped: 131072
> i: 95, status: 0, bytes_deduped: 131072
> i: 96, status: 0, bytes_deduped: 131072
> i: 97, status: 0, bytes_deduped: 131072
> i: 98, status: 0, bytes_deduped: 131072
> i: 99, status: 0, bytes_deduped: 131072
> 13107200 total bytes deduped in this operation
> am: 4.8 MiB (4964352 bytes) converted to sparse holes.
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 072a152355788c767b97e4e4c0e4567720988b84 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> bf00d862c6ad436a1be2be606a8ab88d22166b89 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 0d44cdf030fb149e103cfdc164da3da2b7474c17 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 60831f0e7ffe4b49722612c18685c09f4583b1df am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> a19662b294a3ccdf35dbb18fdd72c62018526d7d am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> 6926a34e0ab3e0a023e8ea85a650f5b4217acab4 am
> ^C
>
> Corruption occurs most often when there is a sequence like this in a file:
>
> ref 1: hole
> ref 2: extent A, offset 0
> ref 3: hole
> ref 4: extent A, offset 8192
>
> This scenario typically arises due to hole-punching or deduplication.
> Hole-punching replaces one extent ref with two references to the same
> extent with a hole between them, so:
>
> ref 1: extent A, offset 0, length 16384
>
> becomes:
>
> ref 1: extent A, offset 0, length 4096
> ref 2: hole, length 8192
> ref 3: extent A, offset 12288, length 4096
>
> Deduplication replaces two distinct extent refs surrounding a hole with
> two references to one of the duplicate extents, turning this:
>
> ref 1: extent A, offset 0, length 4096
> ref 2: hole, length 8192
> ref 3: extent B, offset 0, length 4096
>
> into this:
>
> ref 1: extent A, offset 0, length 4096
> ref 2: hole, length 8192
> ref 3: extent A, offset 0, length 4096
>
> Compression is required (zlib, zstd, or lzo) for corruption to occur.
> I am not able to reproduce the issue with an uncompressed extent nor
> have I observed any such corruption in the wild.
>
> The presence or absence of the no-holes filesystem feature has no effect.
>
> Ordinary writes can lead to pairs of extent references to the same extent
> separated by a reference to a different extent; however, in this case
> there is data to be read from a real extent, instead of pages that have
> to be zero filled from a hole. If ordinary non-hole writes could trigger
> this bug, every page-oriented database engine would be crashing all the
> time on btrfs with compression enabled, and it's unlikely that would not
> have been noticed between 2015 and now. An ordinary write that splits
> an extent ref would look like this:
>
> ref 1: extent A, offset 0, length 4096
> ref 2: extent C, offset 0, length 8192
> ref 3: extent A, offset 12288, length 4096
>
> Sparse writes can lead to pairs of extent references surrounding a hole;
> however, in this case the extent references will point to different
> extents, avoiding the bug. If a sparse write could trigger the bug,
> the rsync -S option and qemu/kvm 'raw' disk image files (among many
> other tools that produce sparse files) would be unusable, and it's
> unlikely that would not have been noticed between 2015 and now either.
> Sparse writes look like this:
>
> ref 1: extent A, offset 0, length 4096
> ref 2: hole, length 8192
> ref 3: extent B, offset 0, length 4096
>
> The pattern or timing of read() calls seems to be relevant. It is very
> hard to see the corruption when reading files with 'hd', but 'cat | hd'
> will see the corruption just fine. Similar problems exist with 'cmp'
> but not 'sha1sum'. Two processes reading the same file at the same time
> seem to trigger the corruption very frequently.
>
> Some patterns of holes and data produce corruption faster than others.
> The pattern generated by the script above is based on instances of
> corruption I've found in the wild, and has a much better repro rate than
> random holes.
>
> The corruption occurs during reads, after csum verification and before
> decompression, so btrfs detects no csum failures. The data on disk
> seems to be OK and could be read correctly once the kernel bug is fixed.
> Repeated reads do eventually return correct data, but there is no way
> for userspace to distinguish between corrupt and correct data reliably.
>
> The corrupted data is usually data replaced by a hole or a copy of other
> blocks in the same extent.
>
> The behavior is similar to some earlier bugs related to holes and
> Compressed data in btrfs, but it's new and not fixed yet--hence,
> "2018 edition."
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