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Subject: [PATCH 05/11] VFS: new function: mount_is_internal()
From:   NeilBrown <neilb@suse.de>
To:     Christoph Hellwig <hch@infradead.org>,
        Josef Bacik <josef@toxicpanda.com>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Chuck Lever <chuck.lever@oracle.com>, Chris Mason <clm@fb.com>,
        David Sterba <dsterba@suse.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>
Cc:     linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org,
        linux-btrfs@vger.kernel.org
Date:   Wed, 28 Jul 2021 08:37:45 +1000
Message-ID: <162742546552.32498.14429836898036234922.stgit@noble.brown>
In-Reply-To: <162742539595.32498.13687924366155737575.stgit@noble.brown>
References: <162742539595.32498.13687924366155737575.stgit@noble.brown>
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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@suse.de>
---
 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);