mm: fix nr_rotate_swap leak in swapon() error case

If swapon() fails after incrementing nr_rotate_swap, we don't decrement
it and thus effectively leak it.  Make sure we decrement it if we
incremented it.

Link: http://lkml.kernel.org/r/b6fe6b879f17fa68eee6cbd876f459f6e5e33495.1526491581.git.osandov@fb.com
Fixes: 81a0298b ("mm, swap: don't use VMA based swap readahead if HDD is used as swap")
Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Rik van Riel <riel@surriel.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swapfile.c

@@ -3112,6 +3112,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	unsigned long *frontswap_map = NULL;
 	struct page *page = NULL;
 	struct inode *inode = NULL;
+	bool inced_nr_rotate_swap = false;
 
 	if (swap_flags & ~SWAP_FLAGS_VALID)
 		return -EINVAL;
@@ -3215,8 +3216,10 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 			cluster = per_cpu_ptr(p->percpu_cluster, cpu);
 			cluster_set_null(&cluster->index);
 		}
-	} else
+	} else {
 		atomic_inc(&nr_rotate_swap);
+		inced_nr_rotate_swap = true;
+	}
 
 	error = swap_cgroup_swapon(p->type, maxpages);
 	if (error)
@@ -3307,6 +3310,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	vfree(swap_map);
 	kvfree(cluster_info);
 	kvfree(frontswap_map);
+	if (inced_nr_rotate_swap)
+		atomic_dec(&nr_rotate_swap);
 	if (swap_file) {
 		if (inode && S_ISREG(inode->i_mode)) {
 			inode_unlock(inode);