Re: Current behaviour of umount on a frozen FS

[Jan Kara <jack@suse.cz>]

On Fri 15-03-13 16:26:39, Carlos Maiolino wrote:
> I'm working on a possible bug related with fsfreeze, where after frozen
> filesystem is umounted the respective block device can't be mounted again,
> returning to userspace, a -EBUSY error.
> 
> Discussing with Eric Sandeen about it, we identified that the current behaviour
> is to make filesystem able to be umounted and mounted again; although it's kept
> frozen (probably a reference to its superblock is pinned in memory once it's
> mounted ?!), we are able to mount it again and thaw it.
> 
> But it raised some questions about what's the expected behaviour of this
> situation, should we allow a frozen filesystem to be umounted? If yes, how about
> the possibility of a snapshot corruption in case a snapshot is running when
> umount is triggered?
  So Al refused to forbid umounting of frozen fs - he maintains that
'umount -l' should always work. I can see his point although in this case
I'm not 100% convinced that the problems this creates aren't worse than
failing umount.

> Since we are able to mount it again, and the current state is the same before
> umount, looks like we keep superblock pinned in memory and data that should have
> generated I/O at umount/mount is kept in memory until the filesystem is thawed.
  Yes. freeze_super() takes active superblock reference and thaw_super()
drops it. So until thaw_super() is called, superblock is still fully alive.
This has a consequence that although the frozen filesystem is umounted,
from fs driver point of view nothing really happens (at least for most
filesystems) since ->put_super() is not called. Only filesystems playing
tricks with ->mount can notice. So we don't have problems with umount
blocking on frozen fs. Similarly mount only attaches live superblock to a
directory hiearchy so again no IO is needed. I'm not sure why mount returns
EBUSY for you (it used to work for me when I was testing it).

> I saw some patches from Fernando in the list but looks like the discussion
> didn't continue.
  Yeah, it's a pity. He did have some useful fixes in his series. It would
be good to revive it and push at least the non-controversial parts (only
the last two patches had some problems).

> What's the current status of this behaviour on fsfreeze? should we allow a
> filesystem to be umounted or not? currently, this is allowed, but, is there
> something protecting snapshots to be corrupted or is this being handled as an
> unlikely situation?
  As I wrote above, snapshots should be fine... It should be all working,
just it is ... somewhat surprising ... from user POV.

									Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR