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From: Josh Triplett <josh@joshtriplett.org>
To: Jan Kara <jack@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Theodore Ts'o <tytso@mit.edu>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	Jan Kara <jack@suse.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-ext4@vger.kernel.org
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.

  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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