linux-kernel - Re: [PATCH] VFS: Add read-only users count to superblock

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Date:	Tue, 14 Jul 2009 16:30:12 -0400
From:	Valerie Aurora <vaurora@...hat.com>
To:	Erez Zadok <ezk@...sunysb.edu>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Jan Blunck <jblunck@...e.de>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] VFS: Add read-only users count to superblock

On Tue, Jul 14, 2009 at 02:18:44PM -0400, Erez Zadok wrote:
> In message <20090713192743.GA27582@...ll>, Valerie Aurora writes:
> > During the last FS summit, Al Viro suggested creating a superblock
> > level read-only marker so that union mounts could guarantee that the
> > underlying fs would not become writable.  This patch implements the
> > VFS support, but doesn't add any users.  The patch making union mounts
> > use the support is in our union mounts tree.  I think we also need
> > some way to pass this through NFS mounts, since a read-only NFS mount
> > for the bottom layer of a union mount is a common use case.
> > 
> > -VAL
> [...]
> 
> Val, I've often wondered if a generic readonly superblock solution will
> obviate the need for the sb->s_vfs_rename_mutex "kludge" (as commented in
> fs.h) and the whole way directory-renames are done wrt locking.  Can the
> rename code be the first user of such patch, or the patch isn't quite ready
> for this?

I'm afraid not!  With this patch, you can only mark the superblock
(and all associated mounts) read-only if no files are open for writing
- not exactly the common case during a directory rename.  So no, it
can't replace the rename mutex, at least in its current form.

-VAL
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