Re: [RFC PATCH v3 14/15] dcache: Implement partial shrink via Slab Movable Objects

[Al Viro <viro@zeniv.linux.org.uk>]

On Thu, Apr 11, 2019 at 10:02:00PM +0100, Al Viro wrote:

> Aaaarrgghhh...  No, we can't.  Look: we get one candidate dentry in isolate
> phase.  We put it into shrink list.  umount(2) comes and calls
> shrink_dcache_for_umount(), which calls shrink_dcache_parent(root).
> In the meanwhile, shrink_dentry_list() is run and does __dentry_kill() on
> that one dentry.  Fine, it's gone - before shrink_dcache_parent() even
> sees it.  Now shrink_dentry_list() holds a reference to its parent and
> is about to drop it in
>                 dentry = parent;
>                 while (dentry && !lockref_put_or_lock(&dentry->d_lockref))
>                         dentry = dentry_kill(dentry);
> And dropped it will be, but... shrink_dcache_parent() has finished the
> scan, without finding *anything* with zero refcount - the thing that used
> to be on the shrink list was already gone before shrink_dcache_parent()
> has gotten there and the reference to parent was not dropped yet.  So
> shrink_dcache_for_umount() plows past shrink_dcache_parent(), walks the
> tree and complains loudly about "busy" dentries (that parent we hadn't
> finished dropping), and then we proceed with filesystem shutdown.
> In the meanwhile, dentry_kill() finally gets to killing dentry and
> triggers an unexpected late call of ->d_iput() on a filesystem that
> has already been far enough into shutdown - far enough to destroy the
> data structures needed for that sucker.
> 
> The reason we don't hit that problem with regular memory shrinker is
> this:
>                 unregister_shrinker(&s->s_shrink);
>                 fs->kill_sb(s);
> in deactivate_locked_super().  IOW, shrinker for this fs is gone
> before we get around to shutdown.  And so are all normal sources
> of dentry eviction for that fs.
> 
> Your earlier variants all suffer the same problem - picking a page
> shared by dentries from several superblocks can run into trouble
> if it overlaps with umount of one of those.

FWIW, I think I see a kinda-sorta sane solution.  Namely, add

static void __dput_to_list(struct dentry *dentry, struct list_head *list)
{
	if (dentry->d_flags & DCACHE_SHRINK_LIST) {
		/* let the owner of the list it's on deal with it */
		--dentry->d_lockref.count;
	} else {
		if (dentry->d_flags & DCACHE_LRU_LIST)
			d_lru_del(dentry);
		if (!--dentry->d_lockref.count)
			d_shrink_add(parent, list);
	}
}

and have
shrink_dentry_list() do this in the end of loop:
                d_shrink_del(dentry);
                parent = dentry->d_parent;
		/* both dentry and parent are locked at that point */
		if (parent != dentry) {
			/*
			 * We need to prune ancestors too. This is necessary to
			 * prevent quadratic behavior of shrink_dcache_parent(),
			 * but is also expected to be beneficial in reducing
			 * dentry cache fragmentation.
			 */
			__dput_to_list(parent, list);
		}
		__dentry_kill(dentry);
        }

instead of
                d_shrink_del(dentry);
                parent = dentry->d_parent;
                __dentry_kill(dentry);
                if (parent == dentry)
                        continue;
                /*
                 * We need to prune ancestors too. This is necessary to prevent
                 * quadratic behavior of shrink_dcache_parent(), but is also
                 * expected to be beneficial in reducing dentry cache
                 * fragmentation.
                 */
                dentry = parent;
                while (dentry && !lockref_put_or_lock(&dentry->d_lockref))
                        dentry = dentry_kill(dentry);
        }
we have there now.  Linus, do you see any problems with that change?  AFAICS,
that should avoid the problem described above.  Moreover, it seems to allow
a fun API addition:

void dput_to_list(struct dentry *dentry, struct list_head *list)
{
	rcu_read_lock();
	if (likely(fast_dput(dentry))) {
		rcu_read_unlock();
		return;
	}
	rcu_read_unlock();
	if (!retain_dentry(dentry))
		__dput_to_list(dentry, list);
	spin_unlock(&dentry->d_lock);
}

allowing to take an empty list, do a bunch of dput_to_list() (under spinlocks,
etc.), then, once we are in better locking conditions, shrink_dentry_list()
to take them all out.  I can see applications for that in e.g. fs/namespace.c -
quite a bit of kludges with ->mnt_ex_mountpoint would be killable that way,
and there would be a chance to transfer the contribution to ->d_count of
mountpoint from struct mount to struct mountpoint (i.e. make any number of
mounts on the same mountpoint dentry contribute only 1 to its ->d_count,
not the number of such mounts).