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SubjectRe: [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@fromorbit.com

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