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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20150409161054.GG25622@twin.jikos.cz>
Date:	Thu, 9 Apr 2015 18:10:54 +0200
From:	David Sterba <dsterba@...e.cz>
To:	Qu Wenruo <quwenruo@...fujitsu.com>
Cc:	Omar Sandoval <osandov@...ndov.com>, Chris Mason <clm@...com>,
	Josef Bacik <jbacik@...com>, David Sterba <dsterba@...e.cz>,
	linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] Btrfs: unify subvol= and subvolid= mounting

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@...ndov.com>
> To: Chris Mason <clm@...com>, Josef Bacik <jbacik@...com>, David Sterba 
> <dsterba@...e.cz>, <linux-btrfs@...r.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.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/