[RFC PATCH] Introduce filesystem type tracking

[Tom Spink]

Hi,

This email contains an RFC patch that introduces init and exit routines to
the file_system_type structure.  These routines were mentioned in
an email I saw about XFS starting threads that aren't needed when no
XFS filesystems are mounted.

So I decided to try and implement the infrastructure to do this.

Please let me know what you think, I'm pretty sure I'll be missing
something I won't know about (like a lock, or a refcount), but feedback
would be appreciated.

--

This patch adds tracking to filesystem types, whereby the number of mounts
of a particular filesystem type can be determined.  This has the added
benefit of introducing init and exit routines for filesystem types, which
are called on the first mount and last unmount of the filesystem type,
respectively.

This is useful for filesystems which share global resources between all
mounts, but only need these resources when at least one filesystem is
mounted.  For example, XFS creates a number of kernel threads which aren't
required when there are no XFS filesystems mounted.  This patch will allow
XFS to start those threads just before the first filesystem is mounted, and
to shut them down when the last filesystem has been unmounted.

Signed-off-by: Tom Spink <tspink@xxxxxxxxx>
---
 fs/namespace.c     |    9 +++++++++
 fs/super.c         |   25 +++++++++++++++++++++++++
 include/linux/fs.h |    3 +++
 3 files changed, 37 insertions(+), 0 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 4fc302c..bfa2f39 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1025,6 +1025,7 @@ static void shrink_submounts(struct vfsmount *mnt, struct list_head *umounts);
 static int do_umount(struct vfsmount *mnt, int flags)
 {
 	struct super_block *sb = mnt->mnt_sb;
+	struct file_system_type *type = sb->s_type;
 	int retval;
 	LIST_HEAD(umount_list);
 
@@ -1108,6 +1109,14 @@ static int do_umount(struct vfsmount *mnt, int flags)
 		security_sb_umount_busy(mnt);
 	up_write(&namespace_sem);
 	release_mounts(&umount_list);
+
+	/* Check to see if the unmount is successful, and we're unmounting the
+	 * last filesystem of this type.  If we are, run the exit routine of
+	 * the filesystem type.
+	 */
+	if (retval == 0 && ((--type->nr_mounts == 0) && type->exit))
+		type->exit();
+
 	return retval;
 }
 
diff --git a/fs/super.c b/fs/super.c
index 453877c..e1dba4b 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -961,14 +961,39 @@ static struct vfsmount *fs_set_subtype(struct vfsmount *mnt, const char *fstype)
 struct vfsmount *
 do_kern_mount(const char *fstype, int flags, const char *name, void *data)
 {
+	int rc;
 	struct file_system_type *type = get_fs_type(fstype);
 	struct vfsmount *mnt;
 	if (!type)
 		return ERR_PTR(-ENODEV);
+
+	/* If this is the first mount, then initialise the filesystem type. */
+	if (type->nr_mounts == 0 && type->init) {
+		rc = type->init();
+
+		/* If initialisation failed, pass the error back down the chain. */
+		if (rc) {
+			put_filesystem(type);
+			return ERR_PTR(rc);
+		}
+	}
+
 	mnt = vfs_kern_mount(type, flags, name, data);
 	if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE) &&
 	    !mnt->mnt_sb->s_subtype)
 		mnt = fs_set_subtype(mnt, fstype);
+
+	/* Check to see if the mount was successful, and if so, increment
+	 * the mount counter.  Otherwise, if we initialised the filesystem
+	 * type already (and the mount just failed), we need to shut it
+	 * back down.
+	 */
+	if (!IS_ERR(mnt)) {
+		type->nr_mounts++;
+	} else if (type->nr_mounts == 0 && type->exit) {
+		type->exit();
+	}
+
 	put_filesystem(type);
 	return mnt;
 }
diff --git a/include/linux/fs.h b/include/linux/fs.h
index f413085..ba92056 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1474,9 +1474,12 @@ int sync_inode(struct inode *inode, struct writeback_control *wbc);
 struct file_system_type {
 	const char *name;
 	int fs_flags;
+	int nr_mounts;
 	int (*get_sb) (struct file_system_type *, int,
 		       const char *, void *, struct vfsmount *);
 	void (*kill_sb) (struct super_block *);
+	int (*init) (void);
+	void (*exit) (void);
 	struct module *owner;
 	struct file_system_type * next;
 	struct list_head fs_supers;
-- 
1.5.4.3

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