Re: nfs-backed mmap file results in 1000s of WRITEs per second

[Jeff Layton <jlayton@redhat.com>]

On Thu, 5 Sep 2013 17:34:20 -0500
Quentin Barnes <qbarnes@gmail.com> wrote:

> On Thu, Sep 05, 2013 at 09:57:24PM +0000, Myklebust, Trond wrote:
> > On Thu, 2013-09-05 at 16:36 -0500, Quentin Barnes wrote:
> > > On Thu, Sep 05, 2013 at 08:02:01PM +0000, Myklebust, Trond wrote:
> > > > On Thu, 2013-09-05 at 14:11 -0500, Quentin Barnes wrote:
> > > > > On Thu, Sep 05, 2013 at 12:03:03PM -0500, Malahal Naineni wrote:
> > > > > > Neil Brown posted a patch couple days ago for this!
> > > > > > 
> > > > > > http://thread.gmane.org/gmane.linux.nfs/58473
> > > > > 
> > > > > I tried Neil's patch on a v3.11 kernel.  The rebuilt kernel still
> > > > > exhibited the same 1000s of WRITEs/sec problem.
> > > > > 
> > > > > Any other ideas?
> > > > 
> > > > Yes. Please try the attached patch.
> > > 
> > > Great!  That did the trick!
> > > 
> > > Do you feel this patch could be worthy of pushing it upstream in its
> > > current state or was it just to verify a theory?
> > > 
> > > 
> > > In comparing the nfs_flush_incompatible() implementations between
> > > RHEL5 and v3.11 (without your patch), the guts of the algorithm seem
> > > more or less logically equivalent to me on whether or not to flush
> > > the page.  Also, when and where nfs_flush_incompatible() is invoked
> > > seems the same.  Would you provide a very brief pointer to clue me
> > > in as to why this problem didn't also manifest circa 2.6.18 days?
> > 
> > There was no nfs_vm_page_mkwrite() to handle page faults in the 2.6.18
> > days, and so the risk was that your mmapped writes could end up being
> > sent with the wrong credentials.
> 
> Ah!  You're right that nfs_vm_page_mkwrite() was missing from
> the original 2.6.18, so that makes sense, however, Red Hat had
> backported that function starting with their RHEL5.9(*) kernels,
> yet the problem doesn't manifest on RHEL5.9.  Maybe the answer lies
> somewhere in RHEL5.9's do_wp_page(), or up that call path, but
> glancing through it, it all looks pretty close though.
> 
> 
> (*) That was the source I using when comparing with the 3.11 source
> when studying your patch since it was the last kernel known to me
> without the problem.
> 

I'm pretty sure RHEL5 has a similar problem, but it's unclear to me why
you're not seeing it there. I have a RHBZ open vs. RHEL5 but it's marked
private at the moment (I'll see about opening it up). I brought this up
upstream about a year ago with this strawman patch:

    http://article.gmane.org/gmane.linux.nfs/51240

...at the time Trond said he was working on a set of patches to track
the open/lock stateid on a per-req basis. Did that approach not pan
out?

Also, do you need to do a similar fix to nfs_can_coalesce_requests?

Thanks,
-- 
Jeff Layton <jlayton@redhat.com>