dcache: use a dispose list in select_parent

select_parent currently abuses the dentry cache LRU to provide
cleanup features for child dentries that need to be freed. It moves
them to the tail of the LRU, then tells shrink_dcache_parent() to
calls __shrink_dcache_sb to unconditionally move them to a dispose
list (as DCACHE_REFERENCED is ignored). __shrink_dcache_sb() has to
relock the dentries to move them off the LRU onto the dispose list,
but otherwise does not touch the dentries that select_parent() moved
to the tail of the LRU. It then passses the dispose list to
shrink_dentry_list() which tries to free the dentries.

IOWs, the use of __shrink_dcache_sb() is superfluous - we can build
exactly the same list of dentries for disposal directly in
select_parent() and call shrink_dentry_list() instead of calling
__shrink_dcache_sb() to do that. This means that we avoid long holds
on the lru lock walking the LRU moving dentries to the dispose list
We also avoid the need to relock each dentry just to move it off the
LRU, reducing the numebr of times we lock each dentry to dispose of
them in shrink_dcache_parent() from 3 to 2 times.

Further, we remove one of the two callers of __shrink_dcache_sb().
This also means that __shrink_dcache_sb can be moved into back into
prune_dcache_sb() and we no longer have to handle referenced
dentries conditionally, simplifying the code.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -276,15 +276,15 @@ static void dentry_lru_prune(struct dentry *dentry)
 	}
 }
 
-static void dentry_lru_move_tail(struct dentry *dentry)
+static void dentry_lru_move_list(struct dentry *dentry, struct list_head *list)
 {
 	spin_lock(&dcache_lru_lock);
 	if (list_empty(&dentry->d_lru)) {
-		list_add_tail(&dentry->d_lru, &dentry->d_sb->s_dentry_lru);
+		list_add_tail(&dentry->d_lru, list);
 		dentry->d_sb->s_nr_dentry_unused++;
 		dentry_stat.nr_unused++;
 	} else {
-		list_move_tail(&dentry->d_lru, &dentry->d_sb->s_dentry_lru);
+		list_move_tail(&dentry->d_lru, list);
 	}
 	spin_unlock(&dcache_lru_lock);
 }
@@ -770,14 +770,18 @@ static void shrink_dentry_list(struct list_head *list)
 }
 
 /**
- * __shrink_dcache_sb - shrink the dentry LRU on a given superblock
- * @sb:		superblock to shrink dentry LRU.
- * @count:	number of entries to prune
- * @flags:	flags to control the dentry processing
+ * prune_dcache_sb - shrink the dcache
+ * @sb: superblock
+ * @count: number of entries to try to free
+ *
+ * Attempt to shrink the superblock dcache LRU by @count entries. This is
+ * done when we need more memory an called from the superblock shrinker
+ * function.
  *
- * If flags contains DCACHE_REFERENCED reference dentries will not be pruned.
+ * This function may fail to free any resources if all the dentries are in
+ * use.
  */
-static void __shrink_dcache_sb(struct super_block *sb, int count, int flags)
+void prune_dcache_sb(struct super_block *sb, int count)
 {
 	struct dentry *dentry;
 	LIST_HEAD(referenced);
@@ -796,13 +800,7 @@ relock:
 			goto relock;
 		}
 
-		/*
-		 * If we are honouring the DCACHE_REFERENCED flag and the
-		 * dentry has this flag set, don't free it.  Clear the flag
-		 * and put it back on the LRU.
-		 */
-		if (flags & DCACHE_REFERENCED &&
-				dentry->d_flags & DCACHE_REFERENCED) {
+		if (dentry->d_flags & DCACHE_REFERENCED) {
 			dentry->d_flags &= ~DCACHE_REFERENCED;
 			list_move(&dentry->d_lru, &referenced);
 			spin_unlock(&dentry->d_lock);
@@ -821,23 +819,6 @@ relock:
 	shrink_dentry_list(&tmp);
 }
 
-/**
- * prune_dcache_sb - shrink the dcache
- * @sb: superblock
- * @nr_to_scan: number of entries to try to free
- *
- * Attempt to shrink the superblock dcache LRU by @nr_to_scan entries. This is
- * done when we need more memory an called from the superblock shrinker
- * function.
- *
- * This function may fail to free any resources if all the dentries are in
- * use.
- */
-void prune_dcache_sb(struct super_block *sb, int nr_to_scan)
-{
-	__shrink_dcache_sb(sb, nr_to_scan, DCACHE_REFERENCED);
-}
-
 /**
  * shrink_dcache_sb - shrink dcache for a superblock
  * @sb: superblock
@@ -1092,7 +1073,7 @@ EXPORT_SYMBOL(have_submounts);
  * drop the lock and return early due to latency
  * constraints.
  */
-static int select_parent(struct dentry * parent)
+static int select_parent(struct dentry *parent, struct list_head *dispose)
 {
 	struct dentry *this_parent;
 	struct list_head *next;
@@ -1114,12 +1095,11 @@ resume:
 
 		spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
 
-		/* 
-		 * move only zero ref count dentries to the end 
-		 * of the unused list for prune_dcache
+		/*
+		 * move only zero ref count dentries to the dispose list.
 		 */
 		if (!dentry->d_count) {
-			dentry_lru_move_tail(dentry);
+			dentry_lru_move_list(dentry, dispose);
 			found++;
 		} else {
 			dentry_lru_del(dentry);
@@ -1181,14 +1161,13 @@ rename_retry:
  *
  * Prune the dcache to remove unused children of the parent dentry.
  */
- 
 void shrink_dcache_parent(struct dentry * parent)
 {
-	struct super_block *sb = parent->d_sb;
+	LIST_HEAD(dispose);
 	int found;
 
-	while ((found = select_parent(parent)) != 0)
-		__shrink_dcache_sb(sb, found, 0);
+	while ((found = select_parent(parent, &dispose)) != 0)
+		shrink_dentry_list(&dispose);
 }
 EXPORT_SYMBOL(shrink_dcache_parent);