From: Josh Triplett <firstname.lastname@example.org> To: Jan Kara <email@example.com> Cc: Linus Torvalds <firstname.lastname@example.org>, Theodore Ts'o <email@example.com>, Andreas Dilger <firstname.lastname@example.org>, Jan Kara <email@example.com>, Linux Kernel Mailing List <firstname.lastname@example.org>, email@example.com Subject: Re: ext4 regression in v5.9-rc2 from e7bfb5c9bb3d on ro fs with overlapped bitmaps Date: Mon, 5 Oct 2020 03:16:41 -0700 [thread overview] Message-ID: <20201005101641.GA516771@localhost> (raw) In-Reply-To: <20201005094601.GB4225@quack2.suse.cz> On Mon, Oct 05, 2020 at 11:46:01AM +0200, Jan Kara wrote: > On Mon 05-10-20 01:14:54, Josh Triplett wrote: > > Ran into an ext4 regression when testing upgrades to 5.9-rc kernels: > > > > Commit e7bfb5c9bb3d ("ext4: handle add_system_zone() failure in > > ext4_setup_system_zone()") breaks mounting of read-only ext4 filesystems > > with intentionally overlapping bitmap blocks. > > > > On an always-read-only filesystem explicitly marked with > > EXT4_FEATURE_RO_COMPAT_SHARED_BLOCKS, prior to that commit, it's safe to > > point all the block and inode bitmaps to a single block of all 1s, > > because a read-only filesystem will never allocate or free any blocks or > > inodes. > > However, after that commit, the block validity check rejects such > > filesystems with -EUCLEAN and "failed to initialize system zone (-117)". > > This causes systems that previously worked correctly to fail when > > upgrading to v5.9-rc2 or later. > > > > This was obviously a bugfix, and I'm not suggesting that it should be > > reverted; it looks like this effectively worked by accident before, > > because the block_validity check wasn't fully functional. However, this > > does break real systems, and I'd like to get some kind of regression fix > > in before 5.9 final if possible. I think it would suffice to make > > block_validity default to false if and only if > > EXT4_FEATURE_RO_COMPAT_SHARED_BLOCKS is set. > > > > Does that seem like a reasonable fix? > > Well, but EXT4_FEATURE_RO_COMPAT_SHARED_BLOCKS is your internal feature > that's not present in current upstream kernel AFAICS. It isn't "my" feature; the value for EXT4_FEATURE_RO_COMPAT_SHARED_BLOCKS is defined in the headers in the e2fsprogs tree. The kernel currently does absolutely nothing with it, nor did it previously need to; it's just an RO_COMPAT feature which ensures that the kernel can only mount the filesystem read-only. The point is that an always-read-only filesystem will never change the block or inode bitmaps, so ensuring they don't overlap is unnecessary (and harmful). I only added EXT4_FEATURE_RO_COMPAT_SHARED_BLOCKS to the header to generate the corresponding ext4_has_feature_shared_blocks function. I have filesystems that previous kernels mounted and worked with just fine, and new kernels reject. That seems like a regression to me. I'm suggesting the simplest possible way I can see to fix that regression. Another approach would be to default block_validity to false for any read-only filesystem mount (since it won't be written to), but that seemed like it'd be more invasive; I was going for a more minimal change.
next prev parent reply other threads:[~2020-10-05 10:16 UTC|newest] Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-10-04 23:17 Linux 5.9-rc8 Linus Torvalds 2020-10-05 8:14 ` ext4 regression in v5.9-rc2 from e7bfb5c9bb3d on ro fs with overlapped bitmaps Josh Triplett 2020-10-05 9:46 ` Jan Kara 2020-10-05 10:16 ` Josh Triplett [this message] 2020-10-05 16:19 ` Jan Kara 2020-10-05 16:20 ` Jan Kara 2020-10-05 17:36 ` Darrick J. Wong 2020-10-06 0:04 ` Theodore Y. Ts'o 2020-10-06 0:32 ` Josh Triplett 2020-10-06 2:51 ` Darrick J. Wong 2020-10-06 3:18 ` Theodore Y. Ts'o 2020-10-06 5:03 ` Josh Triplett 2020-10-06 6:03 ` Josh Triplett 2020-10-06 13:35 ` Theodore Y. Ts'o 2020-10-07 8:03 ` Josh Triplett 2020-10-07 14:32 ` Theodore Y. Ts'o 2020-10-07 20:14 ` Josh Triplett 2020-10-08 2:10 ` Theodore Y. Ts'o 2020-10-08 17:54 ` Darrick J. Wong 2020-10-08 22:38 ` Josh Triplett 2020-10-09 2:54 ` Darrick J. Wong 2020-10-09 19:08 ` Josh Triplett 2020-10-08 22:22 ` Josh Triplett 2020-10-09 14:37 ` Theodore Y. Ts'o 2020-10-09 20:30 ` Josh Triplett 2021-01-10 18:41 ` Malicious fs images was " Pavel Machek 2021-01-11 18:51 ` Darrick J. Wong 2021-01-11 19:39 ` Eric Biggers 2021-01-12 21:43 ` Theodore Ts'o 2021-01-12 22:28 ` Pavel Machek 2021-01-13 5:09 ` Theodore Ts'o 2020-10-08 2:57 ` Andreas Dilger 2020-10-08 19:12 ` Josh Triplett 2020-10-08 19:25 ` Andreas Dilger 2020-10-08 22:28 ` Josh Triplett
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