Re: [BUG REPORT] ext4: “errors=remount-ro” has become “errors=shutdown”?

Jan Kara
2025-01-03T10:42:16Z

[CCed XFS and fsdevel list in case people have opinion what would be the
best interface to know the fs has shutdown]

On Fri 03-01-25 17:26:58, Baokun Li wrote:
> Hi Honza，
>
> Happy New Year!

Thanks!

> On 2025/1/2 23:58, Jan Kara wrote:
> > On Mon 30-12-24 15:27:01, Baokun Li wrote:
> > > We reported similar issues to the community in 2020,
> > > https://lore.kernel.org/all/20210104160457.GG4018@quack2.suse.cz/
> > > Jan Kara provides a simple and effective patch. This patch somehow
> > > didn't end up merged into upstream, but this patch has been merged into
> > > our internal version for a couple years now and it works fine, is it
> > > possible to revert the patch that no longer sets SB_RDONLY and use
> > > the patch in the link above?
> > Well, the problem with filesystem freezing was just the immediate trigger
> > for the changes. But the setting of SB_RDONLY bit by ext4 behind the VFS'
> > back (as VFS is generally in charge of manipulating that bit and you must
> > hold s_umount for that which we cannot get in ext4 when handling errors)
> > was always problematic and I'm almost sure syzbot would find more problems
> > with that than just fs freezing. As such I don't think we should really
> > return to doing that in ext4 but we need to find other ways how to make
> > your error injection to work...
> I believe this is actually a bug in the evolution of the freeze
> functionality. In v2.6.35-rc1 with commit 18e9e5104fcd ("Introduce
> freeze_super and thaw_super for the fsfreeze ioctl"), the introduction
> of freeze_super/thaw_super did not cause any problems when setting the
> irreversible read-only (ro) bit without locking, because at that time
> we used the flag in sb->s_frozen to determine the file system's state.
> It was not until v4.3-rc1 with commit 8129ed29644b ("change sb_writers
> to use percpu_rw_semaphore") introduced locking into
> freeze_super/thaw_super that setting the irreversible ro without locking
> caused thaw_super to fail to release the locks that should have been
> released, eventually leading to hangs or other issues.
>
> Therefore, I believe that the patch discussed in the previous link is
> the correct one, and it has a smaller impact and does not introduce any
> mechanism changes. Furthermore, after roughly reviewing the code using
> SB_RDONLY, I did not find any logic where setting the irreversible ro
> without locking could cause problems. If I have overlooked anything,
> please let me know.

Well, I don't remember the details but I think there were issues when
remount between ro/rw state raced with ext4 error setting the filesystem
read-only. ext4_remount() isn't really prepared for SB_RDONLY changing
under it and VFS could possibly get confused as well. Also if nothing else
the unlocked update (or the properly locked one!) to sb->s_flags can get
lost due to a racing read-modify-write cycle of sb->s_flags from other
process.

All these could possibly be fixed but in general it is a rather fragile
design that the SB_RDONLY flag can change under you at any moment.
Basically two following sb_rdonly() checks have to be prepared to get
different results regardless of locks they hold. And this is very easy to
forget. So I still think moving away from that is a good direction.

> > > What's worse is that after commit
> > >   95257987a638 ("ext4: drop EXT4_MF_FS_ABORTED flag")
> > > was merged in v6.6-rc1, the EXT4_FLAGS_SHUTDOWN bit is set in
> > > ext4_handle_error(). This causes the file system to not be read-only
> > > when an error is triggered in "errors=remount-ro" mode, because
> > > EXT4_FLAGS_SHUTDOWN prevents both writing and reading.
> > Here I don't understand what is really the problem with EXT4_MF_FS_ABORTED
> > removal. What do you exactly mean by "causes the file system to not be
> > read-only"? We still return EROFS where we used to, we disallow writing as
> > we used to. Can you perhaps give an example what changed with this commit?
> Sorry for the lack of clarity in my previous explanation. The key point
> is not about removing EXT4_MF_FS_ABORTED, but rather we will set
> EXT4_FLAGS_SHUTDOWN bit, which not only prevents writes but also prevents
> reads. Therefore, saying it's not read-only actually means it's completely
> unreadable.

Ah, I see. I didn't think about that. Is it that you really want reading to
work from a filesystem after error? Can you share why (I'm just trying to
understand the usecase)? Or is this mostly a theoretical usecase?

I think we could introduce "shutdown modifications" state which would still
allow pure reads to succeed if there's a usecase for such functionality.

> > So how does your framework detect that the filesystem has failed with
> > errors=remount-ro? By parsing /proc/mounts or otherwise querying current
> > filesystem mount options?
> In most cases, run the mount command and filter related options.
> > Would it be acceptable for you to look at some
> > other mount option (e.g. "shutdown") to detect that state? We could easily
> > implement that.
> We do need to add a shutdown hint, but that's not the point.
>
> We've discussed this internally, and now if the logs are flushed,
> we have no way of knowing if the current filesystem is shutdown. We don't
> know if the -EIO from the filesystem is a hardware problem or if the
> filesystem is already shutdown. So even if there is no current problem,
> we should add some kind of hint to let the user know that the current
> filesystem is shutdown.
>
> The changes to display shutdown are as follows, so that we can see if the
> current filesystem has been shutdown in the mount command.

Yes, I think this would be a good addition regardless of other changes we
might need to do. It would be preferable to be able to come up with
something that's acceptable for querying of shutdown state also for other
filesystems - I've CCed fsdevel and XFS in particular since it has much
longer history of fs shutdown implementation.

Honza

> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 3955bec9245d..ba28ef0f662e 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -3157,6 +3157,9 @@ static int _ext4_show_options(struct seq_file *seq,
> struct super_block *sb,
>         if (nodefs && !test_opt(sb, NO_PREFETCH_BLOCK_BITMAPS))
>                 SEQ_OPTS_PUTS("prefetch_block_bitmaps");
>
> +       if (!nodefs && ext4_forced_shutdown(sb))
> +               SEQ_OPTS_PUTS("shutdown");
> +
>         ext4_show_quota_options(seq, sb);
>         return 0;
>  }
> > I'm sorry again for causing you trouble.
>
> Never mind, thank you for your reply!
>
>
> Regards,
> Baokun
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR