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

[Qu Wenruo]

-------- Original Message  --------
Subject: Re: [PATCH 2/3] Btrfs: unify subvol= and subvolid= mounting
From: David Sterba <dsterba@xxxxxxx>
To: Qu Wenruo <quwenruo@xxxxxxxxxxxxxx>
Date: 2015å04æ10æ 00:10

> 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@xxxxxxxxxxx>
> > To: Chris Mason <clm@xxxxxx>, Josef Bacik <jbacik@xxxxxx>, David Sterba
> > <dsterba@xxxxxxx>, <linux-btrfs@xxxxxxxxxxxxxxx>
> > Date: 2015å04æ08æ 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.
Thanks for pointing out this problem.
That's right.

But I found that, since in show_path() function, we can just return 
-EAGAIN without breaking anything, locking in btrfs_path should be enough.

So I can remove all the unneeded lock/sem.

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