linux-ext4 - Re: [kernel-5.11 regression] tune2fs fails after shutdown

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Date:   Thu, 12 Aug 2021 21:38:51 +0800
From:   Boyang Xue <bxue@...hat.com>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu
Subject: Re: [kernel-5.11 regression] tune2fs fails after shutdown

Thanks Jan!

Yes. I will create a test case for fstests.

Thanks,
Boyang

On Thu, Aug 12, 2021 at 8:47 PM Jan Kara <jack@...e.cz> wrote:
>
> Hello Boyang,
>
> On Thu 12-08-21 09:47:30, Boyang Xue wrote:
> > (Adding the author of the commits)
> > Hi Jan,
> >
> > The commit
> >
> > 81414b4dd48 ext4: remove redundant sb checksum recomputation
> >
> > breaks the original reproducer of
> >
> > 4274f516d4bc ext4: recalucate superblock checksum after updating free
> > blocks/inodes
> >
> > I'm wondering is it expected please?
>
> Thanks for report! So for record the problem is not that superblock with
> incorrect checksum would ever get to disk with my patches but the checksum
> will be incorrect in the buffer cache until the moment we start writeout of
> the superblock. And tune2fs accesses the buffer cache and sees the
> incorrect (stale) checksum. It is impossible to fix this problem completely
> (the tune2fs access is fundamentally racy) but yes, I guess returning the
> checksum recalculation back will make the race window small and the cost is
> small. I'll send a patch for this shortly.
>
> Also can you perhaps make this sequence into a fstests testcase for ext4
> filesystem so that we have it covered? Thanks!
>
>                                                                 Honza
>
> > On Thu, Aug 5, 2021 at 10:35 AM Boyang Xue <bxue@...hat.com> wrote:
> > >
> > > Hi,
> > >
> > > kernel commit
> > >
> > > 4274f516d4bc ext4: recalucate superblock checksum after updating free
> > > blocks/inodes
> > >
> > > had been reverted by
> > >
> > > 81414b4dd48 ext4: remove redundant sb checksum recomputation
> > >
> > > since kernel-5.11-rc1. As a result, the original reproducer fails again.
> > >
> > > Reproducer:
> > > ```
> > > mkdir mntpt
> > > fallocate -l 256M mntpt.img
> > > mkfs.ext4 -Fq -t ext4 mntpt.img 128M
> > > LPDEV=$(losetup -f --show mntpt.img)
> > > mount "$LPDEV" mntpt
> > > cp /proc/version mntpt/
> > > ./godown mntpt # godown program attached.
> > > umount mntpt
> > > mount "$LPDEV" mntpt
> > > tune2fs -l "$LPDEV"
> > > ```
> > >
> > > tune2fs fails with
> > > ```
> > > tune2fs 1.46.2 (28-Feb-2021)
> > > tune2fs: Superblock checksum does not match superblock while trying to
> > > open /dev/loop0
> > > Couldn't find valid filesystem superblock.
> > > ```
> > >
> > > Tested on e2fsprogs-1.46.2 + kernel-5.14.0-0.rc3.29. I think it's a
> > > regression. If this is the case, can we fix it again please?
> > >
> > > Thanks,
> > > Boyang
> >
> --
> Jan Kara <jack@...e.com>
> SUSE Labs, CR
>