-------- Original Message --------
Subject: Re: [PATCH RESEND v4 6/8] vfs: Add get_vfsmount_sb() function 
to get vfsmount from a given sb.
From: Miao Xie <redacted>
To: Qu Wenruo <redacted>, <redacted>
Date: 2015年01月30日 08:52

On Thu, 29 Jan 2015 10:24:39 +0800, Qu Wenruo wrote:

quoted

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.

This method is wrong, becasue one fs  may be mounted on the multi places
at the same time, someone is R/O, someone is R/W, you may get a R/O and
fail to get the write permission.

This shouldn't happen. If someone is ro, the whole fs should be ro, right?
You can mount a device which is already mounted as rw to other point as ro,
and remount a mount point to ro will also cause all other mount point to ro.

So I didn't see the problem here.

I think you do label/feature change by sysfs interface by the following way

btrfs_sysfs_change_XXXX()
{
	/* Use trylock to avoid the race with umount */
	if(!mutex_trylock(&sb->s_umount))
		return -EBUSY;

	check R/O and FREEZE

	mutex_unlock(&sb->s_umount);
}

This looks better since it not introduce changes to VFS.

Thanks,
Qu

Thanks
Miao

quoted

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

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;