Re: shrink_dentry_list() logics change (was Re: [RFC PATCH v3 14/15] dcache: Implement partial shrink via Slab Movable Objects)

[Tobin C. Harding]

On Sat, Jun 29, 2019 at 08:06:24PM +0100, Al Viro wrote:
> On Sat, Jun 29, 2019 at 05:38:03AM +0100, Al Viro wrote:
> 
> > PS: the problem is not gone in the next iteration of the patchset in
> > question.  The patch I'm proposing (including dput_to_list() and _ONLY_
> > compile-tested) follows.  Comments?
> 
> FWIW, there's another unpleasantness in the whole thing.  Suppose we have
> picked a page full of dentries, all with refcount 0.  We decide to
> evict all of them.  As it turns out, they are from two filesystems.
> Filesystem 1 is NFS on a server, with currently downed hub on the way
> to it.  Filesystem 2 is local.  We attempt to evict an NFS dentry and
> get stuck - tons of dirty data with no way to flush them on server.
> In the meanwhile, admin tries to unmount the local filesystem.  And
> gets stuck as well, since umount can't do anything to its dentries
> that happen to sit in our shrink list.
> 
> I wonder if the root of problem here isn't in shrink_dcache_for_umount();
> all it really needs is to have everything on that fs with refcount 0
> dragged through __dentry_kill().  If something had been on a shrink
> list, __dentry_kill() will just leave behind a struct dentry completely
> devoid of any connection to superblock, other dentries, filesystem
> type, etc. - it's just a piece of memory that won't be freed until
> the owner of shrink list finally gets around to it.  Which can happen
> at any point - all they'll do to it is dentry_free(), and that doesn't
> need any fs-related data structures.
> 
> The logics in shrink_dcache_parent() is
> 	collect everything evictable into a shrink list
> 	if anything found - kick it out and repeat the scan
> 	otherwise, if something had been on other's shrink list
> 		repeat the scan
> 
> I wonder if after the "no evictable candidates, but something
> on other's shrink lists" we ought to do something along the
> lines of
> 	rcu_read_lock
> 	walk it, doing
> 		if dentry has zero refcount
> 			if it's not on a shrink list,
> 				move it to ours
> 			else
> 				store its address in 'victim'
> 				end the walk
> 	if no victim found
> 		rcu_read_unlock
> 	else
> 		lock victim for __dentry_kill
> 		rcu_read_unlock
> 		if it's still alive
> 			if it's not IS_ROOT
> 				if parent is not on shrink list
> 					decrement parent's refcount
> 					put it on our list
> 				else
> 					decrement parent's refcount
> 			__dentry_kill(victim)
> 		else
> 			unlock
> 	if our list is non-empty
> 		shrink_dentry_list on it
> in there...

Thanks for still thinking about this Al.  I don't have a lot of idea
about what to do with your comments until I can grok them fully but I
wanted to acknowledge having read them.

Thanks,
Tobin.