This patch introduces the concept of an "internal" mount which is a
mount where a filesystem has create the mount itself.
Both the mounted-on-dentry and the mount's root dentry must refer to the
same superblock (they may be the same dentry), and the mounted-on dentry
must be an automount.
Signed-off-by: NeilBrown <neilb@xxxxxxx>
---
fs/namespace.c | 29 +++++++++++++++++++++++++++++
include/linux/mount.h | 2 ++
2 files changed, 31 insertions(+)
diff --git a/fs/namespace.c b/fs/namespace.c
index 73bbdb921e24..a14efbccfb03 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1273,6 +1273,35 @@ bool path_is_mountpoint(const struct path *path)
}
EXPORT_SYMBOL(path_is_mountpoint);
+/**
+ * mount_is_internal() - Check if path is a mount internal to a single filesystem
+ * @mnt: vfsmount to check
+ *
+ * Some filesystems present multiple file-sets using a single
+ * superblock, such as btrfs with multiple subvolumes. Names within a
+ * parent filesystem which lead to a subordinate filesystem are
+ * implemented as automounts so that the structure is visible in the
+ * mount table. nfsd needs visibility into this arrangement so that it
+ * can determine if a mountpoint requires a new export, or is completely
+ * covered by an existing mount.
+ *
+ * An "internal" mount is one where the parent and child have the same
+ * superblock, and the mounted-on dentry is "managed" as an automount. A
+ * filehandle found for an inode in the child can be looked-up using either
+ * vfsmount.
+ */
+bool mount_is_internal(struct vfsmount *mnt)
+{
+ struct mount *m = real_mount(mnt);
+
+ if (!mnt_has_parent(m))
+ return false;
+ if (m->mnt_parent->mnt.mnt_sb != m->mnt.mnt_sb)
+ return false;
+ return m->mnt_mountpoint->d_flags & DCACHE_NEED_AUTOMOUNT;
+}
+EXPORT_SYMBOL(mount_is_internal);
+
struct vfsmount *mnt_clone_internal(const struct path *path)
{
struct mount *p;
diff --git a/include/linux/mount.h b/include/linux/mount.h
index 1d3daed88f83..ab58087728ba 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -118,6 +118,8 @@ extern unsigned int sysctl_mount_max;
extern bool path_is_mountpoint(const struct path *path);
+extern bool mount_is_internal(struct vfsmount *mnt);
+
extern struct vfsmount *lookup_mnt(const struct path *);
extern void kern_unmount_array(struct vfsmount *mnt[], unsigned int num);
