linux-kernel - Re: [patch 1/2] fs: mnt_want

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Date:	Fri, 3 Apr 2009 03:31:46 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [patch 1/2] fs: mnt_want_write speedup

On Thu, Apr 02, 2009 at 07:43:05PM +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.

I guess then it would need another flag to check than checking
RDONLY (BECOMING_RDONLY or something)
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