Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless update of refcount

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

On Sat, Aug 31, 2013 at 03:35:16AM +0100, Al Viro wrote:

> Aha...  OK, I see what's going on.  We end up with shm_mnt *not* marked
> as long-living vfsmount, even though it lives forever.  See if the
> following helps; if it does (and I very much expect it to), we want to
> put it in -stable.  As it is, you get slow path in mntput() each time
> a file created by shmem_file_setup() gets closed.  For no reason whatsoever...

We still want MS_NOUSER on shm_mnt, so we'd better make sure that
shmem_fill_super() sets it on the internal instance...  Fixed variant
follows:

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/mm/shmem.c b/mm/shmem.c
index e43dc55..5261498 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2615,13 +2615,15 @@ int shmem_fill_super(struct super_block *sb, void *data, int silent)
 	 * tmpfs instance, limiting inodes to one per page of lowmem;
 	 * but the internal instance is left unlimited.
 	 */
-	if (!(sb->s_flags & MS_NOUSER)) {
+	if (!(sb->s_flags & MS_KERNMOUNT)) {
 		sbinfo->max_blocks = shmem_default_max_blocks();
 		sbinfo->max_inodes = shmem_default_max_inodes();
 		if (shmem_parse_options(data, sbinfo, false)) {
 			err = -EINVAL;
 			goto failed;
 		}
+	} else {
+		sb->s_flags |= MS_NOUSER;
 	}
 	sb->s_export_op = &shmem_export_ops;
 	sb->s_flags |= MS_NOSEC;
@@ -2831,8 +2833,7 @@ int __init shmem_init(void)
 		goto out2;
 	}
 
-	shm_mnt = vfs_kern_mount(&shmem_fs_type, MS_NOUSER,
-				 shmem_fs_type.name, NULL);
+	shm_mnt = kern_mount(&shmem_fs_type);
 	if (IS_ERR(shm_mnt)) {
 		error = PTR_ERR(shm_mnt);
 		printk(KERN_ERR "Could not kern_mount tmpfs\n");