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

From: Qu Wenruo
Date: Thu Apr 09 2015 - 20:33:19 EST

-------- 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.


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

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