On Tue, Feb 12, 2019 at 3:11 AM Zygo Blaxell
<ce3g8...@umail.furryterror.org> wrote:
>
> Still reproducible on 4.20.7.
I tried your reproducer when you first reported it, on different
machines with different kernel versions.
Never managed to reproduce it, nor see anything obviously wrong in
relevant code paths.
>
> 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."
>
>
--
Filipe David Manana,
“Whether you think you can, or you think you can't — you're right.”
