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

-------- Original Message --------
Subject: Re: [PATCH 2/3] Btrfs: unify subvol= and subvolid= mounting
From: David Sterba <>
To: Qu Wenruo <>
Date: 2015年04月10日 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 <>
>> To: Chris Mason <>, Josef Bacik <>, David Sterba
>> <>, <>
>> 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.
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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