[v4,6/8] vfs: Add get_vfsmount_sb() function to get vfsmount from a given sb.

Commit Message

Qu Wenruo Jan. 29, 2015, 1:27 a.m. UTC

There are sysfs interfaces in some fs, only btrfs yet, which will modify
on-disk data.
Unlike normal file operation routine we can use mnt_want_write_file() to
protect the operation, change through sysfs won't to be binded to any file
in the filesystem.
So we can only extract the first vfsmount of a superblock and pass it to
mnt_want_write() to do the protection.

Cc: linux-fsdevel <[e-mail]>
Signed-off-by: Qu Wenruo <[e-mail]>
---
 fs/namespace.c        | 25 +++++++++++++++++++++++++
 include/linux/mount.h |  1 +
 2 files changed, 26 insertions(+)

Patch

diff --git a/fs/namespace.c b/fs/namespace.c
index cd1e968..5a16a62 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1105,6 +1105,31 @@  struct vfsmount *mntget(struct vfsmount *mnt)
 }
 EXPORT_SYMBOL(mntget);
 
+/*
+ * get a vfsmount from a given sb
+ *
+ * This is especially used for case where change fs' sysfs interface
+ * will lead to a write, e.g. Change label through sysfs in btrfs.
+ * So vfs can get a vfsmount and then use mnt_want_write() to protect.
+ */
+struct vfsmount *get_vfsmount_sb(struct super_block *sb)
+{
+	struct vfsmount *ret_vfs = NULL;
+	struct mount *mnt;
+	int ret = 0;
+
+	lock_mount_hash();
+	if (list_empty(&sb->s_mounts))
+		goto out;
+	mnt = list_entry(sb->s_mounts.next, struct mount, mnt_instance);
+	ret_vfs = &mnt->mnt;
+	ret_vfs = mntget(ret_vfs);
+out:
+	unlock_mount_hash();
+	return ret_vfs;
+}
+EXPORT_SYMBOL(get_vfsmount_sb);
+
 struct vfsmount *mnt_clone_internal(struct path *path)
 {
 	struct mount *p;
diff --git a/include/linux/mount.h b/include/linux/mount.h
index c2c561d..cf1b0f5 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -79,6 +79,7 @@  extern void mnt_drop_write_file(struct file *file);
 extern void mntput(struct vfsmount *mnt);
 extern struct vfsmount *mntget(struct vfsmount *mnt);
 extern struct vfsmount *mnt_clone_internal(struct path *path);
+extern struct vfsmount *get_vfsmount_sb(struct super_block *sb);
 extern int __mnt_is_readonly(struct vfsmount *mnt);
 
 struct path;