On Mon, Mar 26, 2018 at 06:31:51AM +0100, Al Viro wrote: > On Mon, Mar 26, 2018 at 03:35:03PM +1100, Dave Chinner wrote: > > From: Dave Chinner <dchinner@xxxxxxxxxx> > > > > We recently had an oops reported on a 4.14 kernel in > > xfs_reclaim_inodes_count() where sb->s_fs_info pointed to garbage > > and so the m_perag_tree lookup walked into lala land. > > > > We found a mount in a failed state, blocked on teh shrinker rwsem > > here: > > > > mount_bdev() > > deactivate_locked_super() > > unregister_shrinker() > > > > Essentially, the machine was under memory pressure when the mount > > was being run, xfs_fs_fill_super() failed after allocating the > > xfs_mount and attaching it to sb->s_fs_info. It then cleaned up and > > freed the xfs_mount, but the sb->s_fs_info field still pointed to > > the freed memory. Hence when the superblock shrinker then ran > > it fell off the bad pointer. > > > > This is reproduced by using the mount_delay sysfs control as added > > in teh previous patch. It produces an oops down this path during the > > stalled mount: > > > The problem is that the superblock shrinker is running before the > > filesystem structures it depends on have been fully set up. i.e. > > the shrinker is registered in sget(), before ->fill_super() has been > > called, and the shrinker can call into the filesystem before > > fill_super() does it's setup work. > > Wait a sec... How the hell does it get through trylock_super() before > ->s_root is set and ->s_umount is unlocked? I see... So basically the story is * super_cache_count() lacks trylock_super(), making it possible that it'll be called too early on half-set superblock. * it can't be called too late (during fs shutdown), since the shrinker is unregistered before the call of ->kill_sb() * making sure it won't get called too early can be done by checking SB_ACTIVE. It's potentially racy, though - don't we need a barrier between setting the things up and setting SB_ACTIVE? And that, BTW, means that we want SB_BORN instead of SB_ACTIVE - unlike the latter, the former is set only in one place. So I'd suggest switching to checking that, with a barrier pair added (in mount_fs() before setting the sucker, another in super_cache_count() (before doing the scan).
