linux-kernel - Re: [PATCH] vfs: do not try to evict inode when super is frozen

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Message-ID: <YiGgSmEOZUvgmSto@google.com>
Date:   Thu, 3 Mar 2022 21:14:50 -0800
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] vfs: do not try to evict inode when super is frozen

On 03/04, Dave Chinner wrote:
> On Thu, Mar 03, 2022 at 06:21:04PM -0800, Jaegeuk Kim wrote:
> > Otherwise, we will get a deadlock.
> 
> NACK.
> 
> We have to be able to evict clean inodes from memory on frozen
> inodes because we can still instantiate inodes while the filesytem
> is frozen. e.g. there's a find running when the filesystem is
> frozen. What happens if we can't evict clean cached inodes from
> memory when we run out of memory trying to instantiate new inodes?

Ok, that makes sense.

> 
> > 
> > [freeze test]                         shrinkder
> > freeze_super
> >  - pwercpu_down_write(SB_FREEZE_FS)
> >                                        - super_cache_scan
> >                                          - down_read(&sb->s_umount)
> >                                            - prune_icache_sb
> >                                             - dispose_list
> >                                              - evict
> >                                               - f2fs_evict_inode
> > thaw_super
> >  - down_write(&sb->s_umount);
> >                                               - __percpu_down_read(SB_FREEZE_FS)
> 
> That seems like a f2fs bug, not a generic problem.
> 
> Filesystems already have to handle stuff like this if an unlinked
> file is closed while the fs is frozen - we have to handle inode
> eviction needing to modify the file, and different filesystems
> handle this differently. Most filesystems simply block in
> ->evict_inode in this case, but this never occurs from the shrinker
> context.
> 
> IOWs, the shrinker should never be evicting inodes that require the
> filesystem to immediately block on frozen filesystems. If you have
> such inodes in cache at the time the filesystem is frozen, then they
> should be purged from the cache as part of the freeze process so the
> shrinker won't ever find inodes that it could deadlock on.

If so, is this a bug in drop_caches_sysctl_handler? Or, I shouldn't have
used "echo 3 > sysfs/drop_caches" with freezefs in xfstests?

> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@...morbit.com