linux-kernel - Re: [PATCH v3 7/7] NFSD: Pass share reservations flags to VFS

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Date:	Mon, 11 Mar 2013 16:11:08 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Jeff Layton <jlayton@...hat.com>
Cc:	Pavel Shilovsky <piastry@...rsoft.ru>,
	linux-kernel@...r.kernel.org, linux-cifs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
	wine-devel@...ehq.org
Subject: Re: [PATCH v3 7/7] NFSD: Pass share reservations flags to VFS

On Mon, Mar 11, 2013 at 04:08:44PM -0400, Jeff Layton wrote:
> On Mon, 11 Mar 2013 15:36:38 -0400
> "J. Bruce Fields" <bfields@...ldses.org> wrote:
> 
> > On Mon, Mar 11, 2013 at 03:05:40PM -0400, Jeff Layton wrote:
> > > knfsd has some code already to handle share reservations internally.
> > > Nothing outside of knfsd is aware of these reservations, of course so
> > > moving to a vfs-level object for it would be a marked improvement.
> > > 
> > > It doesn't look like this patch removes any of that old code though. I
> > > think it probably should, or there ought to be some consideration of
> > > how this new stuff will mesh with it.
> > > 
> > > I think you have 2 choices here:
> > > 
> > > 1/ rip out the old share reservation code altogether and require that
> > > filesystems mount with -o sharemand or whatever if they want to allow
> > > their enforcement
> > > 
> > > 2/ make knfsd fall back to using the internal share reservation code
> > > when the mount option isn't enabled
> > > 
> > > Personally, I think #1 would be fine, but Bruce may want to weigh in on
> > > what he'd prefer.
> > 
> > #1 sounds good.  Clients that use deny bits are few.  My preference
> > would be to return an error to such clients in the case share locks
> > aren't available.
> > 
> > (We're a little out of spec there, so I'm not sure which error.  I think
> > the goal is to notify a human there's a problem with minimal collateral
> > damange.
> > 
> > NFS4ERR_SERVERFAULT ("I'm a buggy server, sorry about that!") would
> > probably result in an IO error to the application.
> > 
> > SHARE_DENIED strikes me as unsafe: an application would be in its rights
> > not to even check for that e.g. in the case of an exclusive create.
> > 
> > Maybe DELAY?  Kind of ridiculous, but blocking the application
> > indefinitely would probably get someone's attention quickly enough
> > without doing any damnage.)
> > 
> 
> I agree that we should return an error, but hadn't considered what
> error. Given that hardly any NFS clients use them, I'd probably just go
> with NFS4ERR_SERVERFAULT, and maybe throw a printk or something on the
> server about enabling share reservations for superblock x:y.

Sounds reasonable.

> Pavel, as a side note, you may want to consider adding a patch to hook
> this stuff up in the NFS client as well.

Definitely.

--b.
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