Re: [PATCH 2/3] Btrfs: unify subvol= and subvolid= mounting

[David Sterba <dsterba@suse.cz>]

On Wed, Apr 08, 2015 at 02:06:14PM +0800, Qu Wenruo wrote:
> 
> 
> -------- Original Message  --------
> Subject: [PATCH 2/3] Btrfs: unify subvol= and subvolid= mounting
> From: Omar Sandoval <osandov@osandov.com>
> To: Chris Mason <clm@fb.com>, Josef Bacik <jbacik@fb.com>, David Sterba 
> <dsterba@suse.cz>, <linux-btrfs@vger.kernel.org>
> Date: 2015年04月08日 13:34
> 
> > Currently, mounting a subvolume with subvolid= takes a different code
> > path than mounting with subvol=. This isn't really a big deal except for
> > the fact that mounts done with subvolid= or the default subvolume don't
> > have a dentry that's connected to the dentry tree like in the subvol=
> > case. To unify the code paths, when given subvolid= or using the default
> > subvolume ID, translate it into a subvolume name by walking
> > ROOT_BACKREFs in the root tree and INODE_REFs in the filesystem trees.
> Oh, this patch is what I have tried long long ago, and want to do the 
> same thing, to show subvolume mount for btrfs.
> 
> But it came to me that, superblock->show_path() is a better method to do it.
> 
> You can implement btrfs_show_path() to allow mountinfo to get the 
> subvolume name from subvolid, and don't change the mount routine much.

The problem I see with the show_mount approach is related to the
additional path lookup, memory allocation and locking.

If the mountpoint dentry is the right on ,it's just a simple seq_dentry
in show_options.

OTOH, your patch takes subvol_sem that will block the callback if
there's eg. a subvolume being deleted (that takes the write lock). This
is not a lightweight operation nor an infrequent one. There are more
write locks to subvol_sem.

I'm not sure if I've ever sent this comment back to you, sorry if not.