Re: [PATCH RFC v3] vfs: make fstatat retry once on ESTALE errors from getattr call

[Jeff Layton <jlayton-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Fri, 20 Apr 2012 16:18:37 -0400
Steve Dickson <SteveD-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:

> On 04/20/2012 10:40 AM, Jeff Layton wrote:
> > I guess the questions at this point is:
> > 
> > 1) How representative is Peter's mkdir_test() of a real-world workload?
> Reading your email I had to wonder the same thing... What application 
> removes hierarchy of directories in a loop from two different clients?
> I would suspect not many, if any... esp over NFS... 
>  

Peter's test just happens to demonstrate the problem well, but one
could envision someone removing a heirarchy of directories on the
server while we're trying to do other operations in it. At that point,
we can easily end up hitting an ESTALE twice while doing the lookup and
returning ESTALE back to userspace.

> > 
> > 2) if we assume that it is fairly representative of one, how can we
> > achieve retrying indefinitely with NFS, or at least some large finite
> > amount?
> The amount of looping would be peer speculation. If the problem can
> not be handled by one simple retry I would say we simply pass the
> error up to the app... Its an application issue... 
>  

It's not an application issue. The application just asked the kernel
to do an operation on a pathname. The only reason you're getting an
ESTALE back in this situation is a shortcoming of the implementation.

We passed it a pathname after all, not a filehandle. ESTALE really has
no place as a return code in that situation...

> > 
> > I have my doubts as to whether it would really be as big a problem for
> > other filesystems as Miklos and others have asserted, but I'll take
> > their word for it at the moment. What's the best way to contain this
> > behavior to just those filesystems that want to retry indefinitely when
> > they get an ESTALE? Would we need to go with an entirely new
> > ESTALERETRY after all?
> > 
> Introducing a new errno to handle this problem would be overkill IMHO...
> 
> If we have to go to the looping approach, I would strong suggest we
> make the file systems register for this type of behavior...
> 

Returning ESTALERETRY would be registering for it in a way and it is
somewhat cleaner than having to go all the way back up to the fstype to
figure out whether you want to retry it or not.

There's also the non-trivial matter of needing to retry during the
lookup itself. If the lookup just returns ESTALE, then you don't have
any way to know whether the underlying fs wanted you to retry it or
not...

I'm not thrilled with having to do all of this ESTALE to ESTALERETRY
conversion and back, but it does give us a way to neatly deal with and
ESTALE during the lookup.

-- 
Jeff Layton <jlayton-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
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