fs/dcache: Fix incorrect nr_dentry_unused accounting in shrink_dcache_sb()

BugLink: https://bugs.launchpad.net/bugs/1818803

commit 1dbd449c upstream.

The nr_dentry_unused per-cpu counter tracks dentries in both the LRU
lists and the shrink lists where the DCACHE_LRU_LIST bit is set.

The shrink_dcache_sb() function moves dentries from the LRU list to a
shrink list and subtracts the dentry count from nr_dentry_unused.  This
is incorrect as the nr_dentry_unused count will also be decremented in
shrink_dentry_list() via d_shrink_del().

To fix this double decrement, the decrement in the shrink_dcache_sb()
function is taken out.

Fixes: 4e717f5c ("list_lru: remove special case function list_lru_dispose_all."
Cc: stable@kernel.org
Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

fs/dcache.c

@@ -1155,15 +1155,11 @@ static enum lru_status dentry_lru_isolate_shrink(struct list_head *item,
  */
 void shrink_dcache_sb(struct super_block *sb)
 {
-	long freed;
-
 	do {
 		LIST_HEAD(dispose);
 
-		freed = list_lru_walk(&sb->s_dentry_lru,
+		list_lru_walk(&sb->s_dentry_lru,
 			dentry_lru_isolate_shrink, &dispose, 1024);
-
-		this_cpu_sub(nr_dentry_unused, freed);
 		shrink_dentry_list(&dispose);
 		cond_resched();
 	} while (list_lru_count(&sb->s_dentry_lru) > 0);