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Wed, 27 Mar 2019 14:48:16 +0000 (UTC) Subject: Re: [PATCH URGENT v1.1 0/2] btrfs-progs: Fix the nobarrier behavior of write To: Adam Borowski Cc: linux-btrfs@vger.kernel.org References: <20190327094652.16078-1-wqu@suse.com> <20190327140748.GA30466@angband.pl> From: Qu Wenruo Openpgp: preference=signencrypt Autocrypt: addr=wqu@suse.de; prefer-encrypt=mutual; keydata= mQENBFnVga8BCACyhFP3ExcTIuB73jDIBA/vSoYcTyysFQzPvez64TUSCv1SgXEByR7fju3o 8RfaWuHCnkkea5luuTZMqfgTXrun2dqNVYDNOV6RIVrc4YuG20yhC1epnV55fJCThqij0MRL 1NxPKXIlEdHvN0Kov3CtWA+R1iNN0RCeVun7rmOrrjBK573aWC5sgP7YsBOLK79H3tmUtz6b 9Imuj0ZyEsa76Xg9PX9Hn2myKj1hfWGS+5og9Va4hrwQC8ipjXik6NKR5GDV+hOZkktU81G5 gkQtGB9jOAYRs86QG/b7PtIlbd3+pppT0gaS+wvwMs8cuNG+Pu6KO1oC4jgdseFLu7NpABEB AAG0F1F1IFdlbnJ1byA8d3F1QHN1c2UuZGU+iQFUBBMBCAA+AhsDBQsJCAcCBhUICQoLAgQW AgMBAh4BAheAFiEELd9y5aWlW6idqkLhwj2R86El/qgFAlnVgp0FCQlmAm4ACgkQwj2R86El /qilmgf/cUq9kFQo577ku5gc6rFpVg68ublBwjYpwjw0b//xo+Wo1wm+RRbUGs+djSZAqw12 D4F3r0mBTI7abUCNWAbFkYZSAIFVi0DMkjypIVS7PSaEt04rM9VBTToE+YqU6WENeJ57R2p2 +hI0wZrBwxObdsdaOtxWtsp3bmhIbdqxSKrtXuRawy4KnQYcLuGzOce9okdlbAE0W3KHm1gQ oNAe6FX8nC9qo14m8LqEbThYH+qj4iCMlN8HIfbSx4F3e7nHZ+UAMW+E/lnMRkIB9Df+JyVd /NlXzIjZAggcWsqpx6D4wyAuexKWkiGQeUeArUNihAwXjmyqWPGmjVyIh+oC6LkBDQRZ1YGv AQgAqlPrYeBLMv3PAZ75YhQIwH6c4SNcB++hQ9TCT5gIQNw51+SQzkXIGgmzxMIS49cZcE4K Xk/kHw5hieQeQZa60BWVRNXwoRI4ib8okgDuMkD5Kz1WEyO149+BZ7HD4/yK0VFJGuvDJR8T 7RZwB69uVSLjkuNZZmCmDcDzS0c/SJOg5nkxt1iTtgUETb1wNKV6yR9XzRkrEW/qShChyrS9 fNN8e9c0MQsC4fsyz9Ylx1TOY/IF/c6rqYoEEfwnpdlz0uOM1nA1vK+wdKtXluCa79MdfaeD /dt76Kp/o6CAKLLcjU1Iwnkq1HSrYfY3HZWpvV9g84gPwxwxX0uXquHxLwARAQABiQE8BBgB CAAmFiEELd9y5aWlW6idqkLhwj2R86El/qgFAlnVga8CGwwFCQPCZwAACgkQwj2R86El/qgN 8Qf+M0vM2Idwm5txZZSs+/kSgcPxEwYmxUinnUJGyc0ZWYQXPl0cBetZon9El0naijGzNWvf HxIPB+ZFehk6Otgc78p1a3/xck/s1myFRLrmbbTJNoFiyL25ljcq0J8z5Zp4yuABL2RiLdaZ Pt/jfwjBHwGR+QKp6dD2qMrUWf9b7TFzYDMZXzZ2/eoIgtyjEelNBPrIgOFe24iKMjaGjd97 fJuRcBMHdhUAxvXQF1oRtd83JvYJ5OtwTd8MgkEfl+fo7HwWkuHbzc70L4fFKv2BowqFdaHy mId1ijGPGr46tuZ5a4cw/zbaPYx6fJ4sK9tSv/6V1QPNUdqml6hm6pfs6A== Message-ID: Date: Wed, 27 Mar 2019 22:48:12 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.3 MIME-Version: 1.0 In-Reply-To: <20190327140748.GA30466@angband.pl> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="XAkivMCY1bHQOohKDZGOYtbnjCl1KwGmw" Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --XAkivMCY1bHQOohKDZGOYtbnjCl1KwGmw Content-Type: multipart/mixed; boundary="bS1dy1rD2lhuwX4quymv0xzCVDQ3tdSdK"; protected-headers="v1" From: Qu Wenruo To: Adam Borowski Cc: linux-btrfs@vger.kernel.org Message-ID: Subject: Re: [PATCH URGENT v1.1 0/2] btrfs-progs: Fix the nobarrier behavior of write References: <20190327094652.16078-1-wqu@suse.com> <20190327140748.GA30466@angband.pl> In-Reply-To: <20190327140748.GA30466@angband.pl> --bS1dy1rD2lhuwX4quymv0xzCVDQ3tdSdK Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 2019/3/27 =E4=B8=8B=E5=8D=8810:07, Adam Borowski wrote: > On Wed, Mar 27, 2019 at 05:46:50PM +0800, Qu Wenruo wrote: >> This urgent patchset can be fetched from github: >> https://github.com/adam900710/btrfs-progs/tree/flush_super >> Which is based on v4.20.2. >> >> Before this patch, btrfs-progs writes to the fs has no barrier at all.= >> All metadata and superblock are just buffered write, no barrier betwee= n >> super blocks and metadata writes at all. >> >> No wonder why even clear space cache can cause serious transid >> corruption to the originally good fs. >> >> Please merge this fix as soon as possible as I really don't want to se= e >> btrfs-progs corrupting any fs any more. >=20 > How often does this happen in practice? I'm slightly incredulous about= > btrfs-progs crashing often. Especially that pwrite() is buffered on t= he > kernel side, so we'd need a _kernel_ crash (usually a power loss) to br= eak > consistency. Obviously, a potential data loss bug is always something = that > needs fixing, I'm just wondering about severity. Here is a valid case where a crash could cause transid error: - transaction 1 new em at 16K (fs root, gen =3D 1) new em at 32K (extent root, gen =3D 1) new em at 48K (tree root, gen =3D 1) sb->fs root =3D gen 1 sb->extent root =3D gen 1 sb->tree root =3D gen 1 - transaction 2 new em at 64K (extent root, gen =3D 2) new em at 80K (tree root, gen =3D 2) sb->fs root =3D gen 1 at 16K sb->extent root =3D gen 2 sb->tree root =3D gen 2 - transaction 3, half backed due to error commit transaction new eb at 16K (tree root, gen =3D 3) submitted In above case, we will write the newest eb at 16K to disk, but with sb from transaction 2. Then sb expects to read out a tree with gen 1, but get a tree with gen 3.= Further more, even we ignore the generation mismatch, the content of em 16K is completely wrong, super block of gen 2 expects fs root content from em at 16K, but its content is tree root. This should explain the severity much better. Thanks, Qu >=20 > Or do I understand this wrong? >=20 > Asking because Dimitri John Ledkov stepped down as Debian's maintainer = of > this package, and I'm taking up the mantle (with Nicholas D Steeves bei= ng > around) -- modulo any updates other than important bug fixes being on h= old > because of Debian's freeze. Thus, I wonder if this is important enough= to > ask for a freeze exception. >=20 >=20 > Meow! >=20 --bS1dy1rD2lhuwX4quymv0xzCVDQ3tdSdK-- --XAkivMCY1bHQOohKDZGOYtbnjCl1KwGmw Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEELd9y5aWlW6idqkLhwj2R86El/qgFAlybjSwACgkQwj2R86El /qgElwf/Q5i0ZApJZHNQB9jvO4CyWT7tPJsAj5FLyqLkT6oaIsYOBLtnzSqYtRDX ssTbOvkcuoDEaPU+BROnUuUVdY3/AUlLxxkiOIWlGeYppEbMaCoZlJxGDc/McN5D awDX99WrdCzqQ4dRkbkKysDHBUyyCLRMRsWqU8+8NQ3JOlGtqTq8F1KJ7mFx67hX DOshxoOeUAAVdUMa3VAOOIjrul9AppbFMHkPF6cYf152Xj//kNz8BDIvGilY/z28 4SIRisf8XfrIJJW8RVzPRHRYFpx9FyYdja3pN4t+zMQkqMq6Ub2Skl+ZDjezow5Q vfZwFMTIV57Pz4QIovoAwly+zMP7ZA== =IfLQ -----END PGP SIGNATURE----- --XAkivMCY1bHQOohKDZGOYtbnjCl1KwGmw--