From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6225BC282C4 for ; Tue, 12 Feb 2019 17:01:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2423E2084D for ; Tue, 12 Feb 2019 17:01:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729617AbfBLRBL (ORCPT ); Tue, 12 Feb 2019 12:01:11 -0500 Received: from james.kirk.hungrycats.org ([174.142.39.145]:46096 "EHLO james.kirk.hungrycats.org" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1725916AbfBLRBK (ORCPT ); Tue, 12 Feb 2019 12:01:10 -0500 Received: by james.kirk.hungrycats.org (Postfix, from userid 1002) id 450CF2189A2; Tue, 12 Feb 2019 12:01:08 -0500 (EST) Date: Tue, 12 Feb 2019 12:01:03 -0500 From: Zygo Blaxell To: Filipe Manana Cc: linux-btrfs Subject: Re: Reproducer for "compressed data + hole data corruption bug, 2018 edition" still works on 4.20.7 Message-ID: <20190212165916.GA23918@hungrycats.org> References: <20180823031125.GE13528@hungrycats.org> <20190212030838.GB9995@hungrycats.org> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="a8Wt8u1KmwUX3Y2C" Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org --a8Wt8u1KmwUX3Y2C Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Feb 12, 2019 at 03:35:37PM +0000, Filipe Manana wrote: > On Tue, Feb 12, 2019 at 3:11 AM Zygo Blaxell > wrote: > > > > Still reproducible on 4.20.7. >=20 > I tried your reproducer when you first reported it, on different > machines with different kernel versions. That would have been useful to know last August... :-/ > Never managed to reproduce it, nor see anything obviously wrong in > relevant code paths. I built a fresh VM running Debian stretch and reproduced the issue immediately. Mount options are "rw,noatime,compress=3Dzlib,space_cache,subvolid=3D5,subvol=3D/". Kernel is Debian's "4.9.0-8-amd64" but the bug is old enough that kernel version probably doesn't matter. I don't have any configuration that can't reproduce this issue, so I don't know how to help you. I've tested AMD and Intel CPUs, VM, baremetal, hardware ranging in age from 0 to 9 years. Locally built kernels from 4.1 to 4.20 and the stock Debian kernel (4.9). SSDs and spinning rust. All of these reproduce the issue immediately--wrong sha1sum appears in the first 10 loops. What is your test environment? I can try that here. > > > > 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 rea= ds > > > 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=3D{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 wi= th > > > 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 eff= ect. > > > > > > Ordinary writes can lead to pairs of extent references to the same ex= tent > > > 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 ha= ve > > > to be zero filled from a hole. If ordinary non-hole writes could tri= gger > > > this bug, every page-oriented database engine would be crashing all t= he > > > 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 ho= le; > > > 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 ve= ry > > > hard to see the corruption when reading files with 'hd', but 'cat | h= d' > > > will see the corruption just fine. Similar problems exist with 'cmp' > > > but not 'sha1sum'. Two processes reading the same file at the same t= ime > > > 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 t= han > > > 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 fix= ed. > > > Repeated reads do eventually return correct data, but there is no way > > > for userspace to distinguish between corrupt and correct data reliabl= y. > > > > > > The corrupted data is usually data replaced by a hole or a copy of ot= her > > > 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." > > > > >=20 >=20 > --=20 > Filipe David Manana, >=20 > =E2=80=9CWhether you think you can, or you think you can't =E2=80=94 you'= re right.=E2=80=9D >=20 --a8Wt8u1KmwUX3Y2C Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iF0EABECAB0WIQSnOVjcfGcC/+em7H2B+YsaVrMbnAUCXGL7UAAKCRCB+YsaVrMb nOYXAJ9tKZOj4PhQ+ZP7DPlwpkHBrVSs2wCgq1iOBNqjOrjzbyUpcbVaiB392LI= =ykI2 -----END PGP SIGNATURE----- --a8Wt8u1KmwUX3Y2C--