Re: ext4 regression in v5.9-rc2 from e7bfb5c9bb3d on ro fs with overlapped bitmaps

[Josh Triplett]

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.