Re: [patch 1/2] fs: mnt_want_write speedup

[Dave Hansen <dave@linux.vnet.ibm.com>]

On Thu, 2009-04-02 at 19:43 +0100, Al Viro wrote:
> On Thu, Apr 02, 2009 at 08:22:10PM +0200, Nick Piggin wrote:
> > On Wed, Mar 18, 2009 at 12:13:43PM -0700, Dave Hansen wrote:
> > > On Thu, 2009-03-12 at 05:13 +0100, Nick Piggin wrote: 
> > > > On Wed, Mar 11, 2009 at 03:11:17PM -0700, Dave Hansen wrote:
> > > > > I'm feeling a bit better about these, although I am still honestly quite
> > > > > afraid of the barriers.  I also didn't like all the #ifdefs much, but
> > > > > here's some help on that.
> > > > 
> > > > FWIW, we have this in suse kernels because page fault performance was
> > > > so bad compared with SLES10. mnt_want_write & co was I think the 2nd
> > > > biggest offender for file backed mappings (after pvops). I think we're
> > > > around parity again even with pvops.
> > > 
> > > Page faults themselves?  Which path was that from?
> > 
> > Yes. file_update_time.
> 
> FWIW, I'm not sure that this optimization is valid.  We might eventually
> want to go for "don't allow any new writers, remount r/o when existing
> ones expire" functionality, so nested mnt_want_write() might eventually
> be allowed to fail.

That makes sense on a larger scale definitely.

But I do wonder about file_update_time() specifically, especially since
its mnt_want_write() is never persistent and it is always done under the
cover of a FMODE_WRITE 'struct file'.  Do we strictly even need the
mnt_want/drop_write() pair in here at all right now?

-- Dave