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

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

On Sat, Jun 29, 2019 at 05:08:44AM +0100, Al Viro wrote:
> > 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.

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?

diff --git a/fs/dcache.c b/fs/dcache.c
index 8136bda27a1f..dfe21a649c96 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -860,6 +860,32 @@ void dput(struct dentry *dentry)
 }
 EXPORT_SYMBOL(dput);
 
+static void __dput_to_list(struct dentry *dentry, struct list_head *list)
+__must_hold(&dentry->d_lock)
+{
+	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(dentry, list);
+	}
+}
+
+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);
+}
 
 /* This must be called with d_lock held */
 static inline void __dget_dlock(struct dentry *dentry)
@@ -1088,18 +1114,9 @@ static void shrink_dentry_list(struct list_head *list)
 		rcu_read_unlock();
 		d_shrink_del(dentry);
 		parent = dentry->d_parent;
+		if (parent != dentry)
+			__dput_to_list(parent, list);
 		__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);
 	}
 }