vfs: fix dentry LRU list handling and nr_dentry_unused accounting

The LRU list changes interacted badly with our nr_dentry_unused
accounting, and even worse with the new DCACHE_LRU_LIST bit logic.

This introduces helper functions to make sure everything follows the
proper dcache d_lru list rules: the dentry cache is complicated by the
fact that some of the hotpaths don't even want to look at the LRU list
at all, and the fact that we use the same list entry in the dentry for
both the LRU list and for our temporary shrinking lists when removing
things from the LRU.

The helper functions temporarily have some extra sanity checking for the
flag bits that have to match the current LRU state of the dentry.  We'll
remove that before the final 3.12 release, but considering how easy it
is to get wrong, this first cleanup version has some very particular
sanity checking.
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/dcache.c

@@ -356,16 +356,81 @@ static void dentry_unlink_inode(struct dentry * dentry)
 		iput(inode);
 }
 
+/*
+ * The DCACHE_LRU_LIST bit is set whenever the 'd_lru' entry
+ * is in use - which includes both the "real" per-superblock
+ * LRU list _and_ the DCACHE_SHRINK_LIST use.
+ *
+ * The DCACHE_SHRINK_LIST bit is set whenever the dentry is
+ * on the shrink list (ie not on the superblock LRU list).
+ *
+ * The per-cpu "nr_dentry_unused" counters are updated with
+ * the DCACHE_LRU_LIST bit.
+ *
+ * These helper functions make sure we always follow the
+ * rules. d_lock must be held by the caller.
+ */
+#define D_FLAG_VERIFY(dentry,x) WARN_ON_ONCE(((dentry)->d_flags & (DCACHE_LRU_LIST | DCACHE_SHRINK_LIST)) != (x))
+static void d_lru_add(struct dentry *dentry)
+{
+	D_FLAG_VERIFY(dentry, 0);
+	dentry->d_flags |= DCACHE_LRU_LIST;
+	this_cpu_inc(nr_dentry_unused);
+	WARN_ON_ONCE(!list_lru_add(&dentry->d_sb->s_dentry_lru, &dentry->d_lru));
+}
+
+static void d_lru_del(struct dentry *dentry)
+{
+	D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
+	dentry->d_flags &= ~DCACHE_LRU_LIST;
+	this_cpu_dec(nr_dentry_unused);
+	WARN_ON_ONCE(!list_lru_del(&dentry->d_sb->s_dentry_lru, &dentry->d_lru));
+}
+
+static void d_shrink_del(struct dentry *dentry)
+{
+	D_FLAG_VERIFY(dentry, DCACHE_SHRINK_LIST | DCACHE_LRU_LIST);
+	list_del_init(&dentry->d_lru);
+	dentry->d_flags &= ~(DCACHE_SHRINK_LIST | DCACHE_LRU_LIST);
+	this_cpu_dec(nr_dentry_unused);
+}
+
+static void d_shrink_add(struct dentry *dentry, struct list_head *list)
+{
+	D_FLAG_VERIFY(dentry, 0);
+	list_add(&dentry->d_lru, list);
+	dentry->d_flags |= DCACHE_SHRINK_LIST | DCACHE_LRU_LIST;
+	this_cpu_inc(nr_dentry_unused);
+}
+
+/*
+ * These can only be called under the global LRU lock, ie during the
+ * callback for freeing the LRU list. "isolate" removes it from the
+ * LRU lists entirely, while shrink_move moves it to the indicated
+ * private list.
+ */
+static void d_lru_isolate(struct dentry *dentry)
+{
+	D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
+	dentry->d_flags &= ~DCACHE_LRU_LIST;
+	this_cpu_dec(nr_dentry_unused);
+	list_del_init(&dentry->d_lru);
+}
+
+static void d_lru_shrink_move(struct dentry *dentry, struct list_head *list)
+{
+	D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
+	dentry->d_flags |= DCACHE_SHRINK_LIST;
+	list_move_tail(&dentry->d_lru, list);
+}
+
 /*
  * dentry_lru_(add|del)_list) must be called with d_lock held.
  */
 static void dentry_lru_add(struct dentry *dentry)
 {
-	if (unlikely(!(dentry->d_flags & DCACHE_LRU_LIST))) {
-		if (list_lru_add(&dentry->d_sb->s_dentry_lru, &dentry->d_lru))
-			this_cpu_inc(nr_dentry_unused);
-		dentry->d_flags |= DCACHE_LRU_LIST;
-	}
+	if (unlikely(!(dentry->d_flags & DCACHE_LRU_LIST)))
+		d_lru_add(dentry);
 }
 
 /*
@@ -377,15 +442,11 @@ static void dentry_lru_add(struct dentry *dentry)
  */
 static void dentry_lru_del(struct dentry *dentry)
 {
-	if (dentry->d_flags & DCACHE_SHRINK_LIST) {
-		list_del_init(&dentry->d_lru);
-		dentry->d_flags &= ~DCACHE_SHRINK_LIST;
-		return;
+	if (dentry->d_flags & DCACHE_LRU_LIST) {
+		if (dentry->d_flags & DCACHE_SHRINK_LIST)
+			return d_shrink_del(dentry);
+		d_lru_del(dentry);
 	}
-
-	if (list_lru_del(&dentry->d_sb->s_dentry_lru, &dentry->d_lru))
-		this_cpu_dec(nr_dentry_unused);
-	dentry->d_flags &= ~DCACHE_LRU_LIST;
 }
 
 /**
@@ -837,6 +898,12 @@ static void shrink_dentry_list(struct list_head *list)
 		dentry = list_entry_rcu(list->prev, struct dentry, d_lru);
 		if (&dentry->d_lru == list)
 			break; /* empty */
+
+		/*
+		 * Get the dentry lock, and re-verify that the dentry is
+		 * this on the shrinking list. If it is, we know that
+		 * DCACHE_SHRINK_LIST and DCACHE_LRU_LIST are set.
+		 */
 		spin_lock(&dentry->d_lock);
 		if (dentry != list_entry(list->prev, struct dentry, d_lru)) {
 			spin_unlock(&dentry->d_lock);
@@ -848,8 +915,7 @@ static void shrink_dentry_list(struct list_head *list)
 		 * to the LRU here, so we can simply remove it from the list
 		 * here regardless of whether it is referenced or not.
 		 */
-		list_del_init(&dentry->d_lru);
-		dentry->d_flags &= ~DCACHE_SHRINK_LIST;
+		d_shrink_del(dentry);
 
 		/*
 		 * We found an inuse dentry which was not removed from
@@ -861,12 +927,20 @@ static void shrink_dentry_list(struct list_head *list)
 		}
 		rcu_read_unlock();
 
+		/*
+		 * If 'try_to_prune()' returns a dentry, it will
+		 * be the same one we passed in, and d_lock will
+		 * have been held the whole time, so it will not
+		 * have been added to any other lists. We failed
+		 * to get the inode lock.
+		 *
+		 * We just add it back to the shrink list.
+		 */
 		dentry = try_prune_one_dentry(dentry);
 
 		rcu_read_lock();
 		if (dentry) {
-			dentry->d_flags |= DCACHE_SHRINK_LIST;
-			list_add(&dentry->d_lru, list);
+			d_shrink_add(dentry, list);
 			spin_unlock(&dentry->d_lock);
 		}
 	}
@@ -894,7 +968,7 @@ dentry_lru_isolate(struct list_head *item, spinlock_t *lru_lock, void *arg)
 	 * another pass through the LRU.
 	 */
 	if (dentry->d_lockref.count) {
-		list_del_init(&dentry->d_lru);
+		d_lru_isolate(dentry);
 		spin_unlock(&dentry->d_lock);
 		return LRU_REMOVED;
 	}
@@ -925,9 +999,7 @@ dentry_lru_isolate(struct list_head *item, spinlock_t *lru_lock, void *arg)
 		return LRU_ROTATE;
 	}
 
-	dentry->d_flags |= DCACHE_SHRINK_LIST;
-	list_move_tail(&dentry->d_lru, freeable);
-	this_cpu_dec(nr_dentry_unused);
+	d_lru_shrink_move(dentry, freeable);
 	spin_unlock(&dentry->d_lock);
 
 	return LRU_REMOVED;
@@ -972,9 +1044,7 @@ static enum lru_status dentry_lru_isolate_shrink(struct list_head *item,
 	if (!spin_trylock(&dentry->d_lock))
 		return LRU_SKIP;
 
-	dentry->d_flags |= DCACHE_SHRINK_LIST;
-	list_move_tail(&dentry->d_lru, freeable);
-	this_cpu_dec(nr_dentry_unused);
+	d_lru_shrink_move(dentry, freeable);
 	spin_unlock(&dentry->d_lock);
 
 	return LRU_REMOVED;
@@ -1362,9 +1432,13 @@ static enum d_walk_ret select_collect(void *_data, struct dentry *dentry)
 	if (dentry->d_lockref.count) {
 		dentry_lru_del(dentry);
 	} else if (!(dentry->d_flags & DCACHE_SHRINK_LIST)) {
-		dentry_lru_del(dentry);
-		list_add_tail(&dentry->d_lru, &data->dispose);
-		dentry->d_flags |= DCACHE_SHRINK_LIST;
+		/*
+		 * We can't use d_lru_shrink_move() because we
+		 * need to get the global LRU lock and do the
+		 * RLU accounting.
+		 */
+		d_lru_del(dentry);
+		d_shrink_add(dentry, &data->dispose);
 		data->found++;
 		ret = D_WALK_NORETRY;
 	}