saner calling conventions for ->d_automount()

Currently the calling conventions for ->d_automount() instances have
an odd wart - returned new mount to be attached is expected to have
refcount 2.

That kludge is intended to make sure that mark_mounts_for_expiry() called
before we get around to attaching that new mount to the tree won't decide
to take it out.  finish_automount() drops the extra reference after it's
done with attaching mount to the tree - or drops the reference twice in
case of error.  ->d_automount() instances have rather counterintuitive
boilerplate in them.

There's a much simpler approach: have mark_mounts_for_expiry() skip the
mounts that are yet to be mounted.  And to hell with grabbing/dropping
those extra references.  Makes for simpler correctness analysis, at that...

Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Acked-by: David Howells <dhowells@redhat.com>
Tested-by: David Howells <dhowells@redhat.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Author: Al Viro

Documentation/filesystems/porting.rst

@@ -1203,3 +1203,10 @@ should use d_drop();d_splice_alias() and return the result of the latter.
 If a positive dentry cannot be returned for some reason, in-kernel
 clients such as cachefiles, nfsd, smb/server may not perform ideally but
 will fail-safe.
+
+---
+
+**mandatory**
+
+Calling conventions for ->d_automount() have changed; we should *not* grab
+an extra reference to new mount - it should be returned with refcount 1.

Documentation/filesystems/vfs.rst

@@ -1411,9 +1411,7 @@ defined:
 
 	If a vfsmount is returned, the caller will attempt to mount it
 	on the mountpoint and will remove the vfsmount from its
-	expiration list in the case of failure.  The vfsmount should be
-	returned with 2 refs on it to prevent automatic expiration - the
-	caller will clean up the additional ref.
+	expiration list in the case of failure.
 
 	This function is only used if DCACHE_NEED_AUTOMOUNT is set on
 	the dentry.  This is set by __d_instantiate() if S_AUTOMOUNT is

fs/afs/mntpt.c

@@ -189,7 +189,6 @@ struct vfsmount *afs_d_automount(struct path *path)
 	if (IS_ERR(newmnt))
 		return newmnt;
 
-	mntget(newmnt); /* prevent immediate expiration */
 	mnt_set_expiry(newmnt, &afs_vfsmounts);
 	queue_delayed_work(afs_wq, &afs_mntpt_expiry_timer,
 			   afs_mntpt_expiry_timeout * HZ);

fs/fuse/dir.c

@@ -319,9 +319,6 @@ static struct vfsmount *fuse_dentry_automount(struct path *path)
 
 	/* Create the submount */
 	mnt = fc_mount(fsc);
-	if (!IS_ERR(mnt))
-		mntget(mnt);
-
 	put_fs_context(fsc);
 	return mnt;
 }

fs/namespace.c

@@ -3902,10 +3902,6 @@ int finish_automount(struct vfsmount *m, const struct path *path)
 		return PTR_ERR(m);
 
 	mnt = real_mount(m);
-	/* The new mount record should have at least 2 refs to prevent it being
-	 * expired before we get a chance to add it
-	 */
-	BUG_ON(mnt_get_count(mnt) < 2);
 
 	if (m->mnt_sb == path->mnt->mnt_sb &&
 	    m->mnt_root == dentry) {
@@ -3938,7 +3934,6 @@ int finish_automount(struct vfsmount *m, const struct path *path)
 	unlock_mount(mp);
 	if (unlikely(err))
 		goto discard;
-	mntput(m);
 	return 0;
 
 discard_locked:
@@ -3952,7 +3947,6 @@ int finish_automount(struct vfsmount *m, const struct path *path)
 		namespace_unlock();
 	}
 	mntput(m);
-	mntput(m);
 	return err;
 }
 
@@ -3989,11 +3983,14 @@ void mark_mounts_for_expiry(struct list_head *mounts)
 
 	/* extract from the expiration list every vfsmount that matches the
 	 * following criteria:
+	 * - already mounted
 	 * - only referenced by its parent vfsmount
 	 * - still marked for expiry (marked on the last call here; marks are
 	 *   cleared by mntput())
 	 */
 	list_for_each_entry_safe(mnt, next, mounts, mnt_expire) {
+		if (!is_mounted(&mnt->mnt))
+			continue;
 		if (!xchg(&mnt->mnt_expiry_mark, 1) ||
 			propagate_mount_busy(mnt, 1))
 			continue;

fs/nfs/namespace.c

@@ -195,7 +195,6 @@ struct vfsmount *nfs_d_automount(struct path *path)
 	if (IS_ERR(mnt))
 		goto out_fc;
 
-	mntget(mnt); /* prevent immediate expiration */
 	if (timeout <= 0)
 		goto out_fc;
 

fs/smb/client/namespace.c

@@ -283,7 +283,6 @@ struct vfsmount *cifs_d_automount(struct path *path)
 		return newmnt;
 	}
 
-	mntget(newmnt); /* prevent immediate expiration */
 	mnt_set_expiry(newmnt, &cifs_automount_list);
 	schedule_delayed_work(&cifs_automount_task,
 			      cifs_mountpoint_expiry_timeout);

kernel/trace/trace.c

@@ -10088,8 +10088,6 @@ static struct vfsmount *trace_automount(struct dentry *mntpt, void *ingore)
 	put_filesystem(type);
 	if (IS_ERR(mnt))
 		return NULL;
-	mntget(mnt);
-
 	return mnt;
 }