[PATCH] Allow kernel-only mount interfaces...

[Trond Myklebust <trond.myklebust@fys.uio.no>]

Hi,

  I'm working on straightening out a problem we have with NFSv4 in the
case where the server exports a tree of filesystems (as we also do in
NFSv2/v3 with the "nohide" option).

  If the client mounts such a tree, it is supposed to detect whenever it
crosses a mountpoint on the server, and basically automount the new
filesystem in-place. The main reason for wanting to do so is that NFSv4
allows for individual filesystems to be migrated and/or replicated to
other servers.

  In order to respect atomicity guarantees, it would be nice to do this
automounting in-kernel within nfs_lookup(). For efficiency reasons, it
would be nice to be able to pass the old superblock as a parameter to
the mount code. The problem is that the current mount interfaces do not
allow the kernel to have a private api that differs from the standard
userland mount.

  Would something like the appended patch be an acceptable method of
implementing such a private mount api?

Cheers,
  Trond

VFS: Add GPL_EXPORTED function vfs_kern_mount()

 do_kern_mount() does not allow the kernel to use private mount interfaces
 without exposing the same interfaces to userland. The problem is that the
 filesystem is referenced by name, thus meaning that it and its mount
 interface must be registered in the global filesystem list.

 vfs_kern_mount() passes the struct file_system_type as an explicit
 parameter in order to overcome this limitation.

 Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
---
 super.c |   16 ++++++++++++----
 1 files changed, 12 insertions(+), 4 deletions(-)

Index: linux-2.6.11-rc3/fs/super.c
===================================================================
--- linux-2.6.11-rc3.orig/fs/super.c
+++ linux-2.6.11-rc3/fs/super.c
@@ -794,9 +794,8 @@ struct super_block *get_sb_single(struct
 EXPORT_SYMBOL(get_sb_single);
 
 struct vfsmount *
-do_kern_mount(const char *fstype, int flags, const char *name, void *data)
+vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void *data)
 {
-	struct file_system_type *type = get_fs_type(fstype);
 	struct super_block *sb = ERR_PTR(-ENOMEM);
 	struct vfsmount *mnt;
 	int error;
@@ -835,7 +834,6 @@ do_kern_mount(const char *fstype, int fl
 	mnt->mnt_parent = mnt;
 	mnt->mnt_namespace = current->namespace;
 	up_write(&sb->s_umount);
-	put_filesystem(type);
 	return mnt;
 out_sb:
 	up_write(&sb->s_umount);
@@ -846,10 +844,20 @@ out_free_secdata:
 out_mnt:
 	free_vfsmnt(mnt);
 out:
-	put_filesystem(type);
 	return (struct vfsmount *)sb;
 }
 
+EXPORT_SYMBOL_GPL(vfs_kern_mount);
+
+struct vfsmount *
+do_kern_mount(const char *fstype, int flags, const char *name, void *data)
+{
+	struct file_system_type *type = get_fs_type(fstype);
+	struct vfsmount *mnt = vfs_kern_mount(type, flags, name, data);
+	put_filesystem(type);
+	return mnt;
+}
+
 EXPORT_SYMBOL_GPL(do_kern_mount);
 
 struct vfsmount *kern_mount(struct file_system_type *type)


-- 
Trond Myklebust <trond.myklebust@fys.uio.no>