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

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Message-ID: <20090402184305.GI28946@ZenIV.linux.org.uk>
Date:	Thu, 2 Apr 2009 19:43:05 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Nick Piggin <npiggin@...e.de>
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 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.
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