Current behaviour of umount on a frozen FS

[Carlos Maiolino <cmaiolino@redhat.com>]

Hi guys, first, people Cc'ed on the email were also Cc'ed on a thread related to
a similar problem.

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?

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.

I saw some patches from Fernando in the list but looks like the discussion
didn't continue.

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?

Cheers,
-- 
Carlos