VFS : mount lock scalability for internal mounts

For a number of file systems that don't have a mount point (e.g. sockfs
and pipefs), they are not marked as long term. Therefore in
mntput_no_expire, all locks in vfs_mount lock are taken instead of just
local cpu's lock to aggregate reference counts when we release
reference to file objects.  In fact, only local lock need to have been
taken to update ref counts as these file systems are in no danger of
going away until we are ready to unregister them.

The attached patch marks file systems using kern_mount without
mount point as long term.  The contentions of vfs_mount lock
is now eliminated.  Before un-registering such file system,
kern_unmount should be called to remove the long term flag and
make the mount point ready to be freed.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

7 files changed, 26 insertions, 4 deletions

diff --git a/drivers/mtd/mtdchar.c b/drivers/mtd/mtdchar.c
index 3f92731a5b9e..f1af2228a1b1 100644
--- a/drivers/mtd/mtdchar.c
+++ b/drivers/mtd/mtdchar.c
@@ -1192,7 +1192,7 @@ err_unregister_chdev:
 static void __exit cleanup_mtdchar(void)
 {
         unregister_mtd_user(&mtdchar_notifier);
-        mntput(mtd_inode_mnt);
+        kern_unmount(mtd_inode_mnt);
         unregister_filesystem(&mtd_inodefs_type);
         __unregister_chrdev(MTD_CHAR_MAJOR, 0, 1 << MINORBITS, "mtd");
 }
diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
index c5567cb78432..4d433d34736f 100644
--- a/fs/anon_inodes.c
+++ b/fs/anon_inodes.c
@@ -233,7 +233,7 @@ static int __init anon_inode_init(void)
         return 0;
 
 err_mntput:
-        mntput(anon_inode_mnt);
+        kern_unmount(anon_inode_mnt);
 err_unregister_filesystem:
         unregister_filesystem(&anon_inode_fs_type);
 err_exit:
diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 7aafeb8fa300..0b686cec9976 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -1030,6 +1030,7 @@ static int __init init_hugetlbfs_fs(void)
 static void __exit exit_hugetlbfs_fs(void)
 {
         kmem_cache_destroy(hugetlbfs_inode_cachep);
+        kern_unmount(hugetlbfs_vfsmount);
         unregister_filesystem(&hugetlbfs_fs_type);
         bdi_destroy(&hugetlbfs_backing_dev_info);
 }
diff --git a/fs/namespace.c b/fs/namespace.c
index cda50fe9250a..22bfe8273c68 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2721,6 +2721,25 @@ EXPORT_SYMBOL(put_mnt_ns);
 
 struct vfsmount *kern_mount_data(struct file_system_type *type, void *data)
 {
-        return vfs_kern_mount(type, MS_KERNMOUNT, type->name, data);
+        struct vfsmount *mnt;
+        mnt = vfs_kern_mount(type, MS_KERNMOUNT, type->name, data);
+        if (!IS_ERR(mnt)) {
+                /*
+                 * it is a longterm mount, don't release mnt until
+                 * we unmount before file sys is unregistered
+                */
+                mnt_make_longterm(mnt);
+        }
+        return mnt;
 }
 EXPORT_SYMBOL_GPL(kern_mount_data);
+
+void kern_unmount(struct vfsmount *mnt)
+{
+        /* release long term mount so mount point can be released */
+        if (!IS_ERR_OR_NULL(mnt)) {
+                mnt_make_shortterm(mnt);
+                mntput(mnt);
+        }
+}
+EXPORT_SYMBOL(kern_unmount);
diff --git a/fs/pipe.c b/fs/pipe.c
index da42f7db50de..1b7f9af67ccf 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -1291,8 +1291,8 @@ static int __init init_pipe_fs(void)
 
 static void __exit exit_pipe_fs(void)
 {
+        kern_unmount(pipe_mnt);
         unregister_filesystem(&pipe_fs_type);
-        mntput(pipe_mnt);
 }
 
 fs_initcall(init_pipe_fs);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index b224dc468a23..7a757a48a5c6 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1885,6 +1885,7 @@ extern int register_filesystem(struct file_system_type *);
 extern int unregister_filesystem(struct file_system_type *);
 extern struct vfsmount *kern_mount_data(struct file_system_type *, void *data);
 #define kern_mount(type) kern_mount_data(type, NULL)
+extern void kern_unmount(struct vfsmount *mnt);
 extern int may_umount_tree(struct vfsmount *);
 extern int may_umount(struct vfsmount *);
 extern long do_mount(char *, char *, char *, unsigned long, void *);
diff --git a/security/selinux/selinuxfs.c b/security/selinux/selinuxfs.c
index 35459340019e..de7900ef53da 100644
--- a/security/selinux/selinuxfs.c
+++ b/security/selinux/selinuxfs.c
@@ -1984,6 +1984,7 @@ __initcall(init_sel_fs);
 void exit_sel_fs(void)
 {
         kobject_put(selinuxfs_kobj);
+        kern_unmount(selinuxfs_mount);
         unregister_filesystem(&sel_fs_type);
 }
 #endif