Date: Thu, 2 Apr 2009 19:43:05 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Nick Piggin <npiggin@suse.de>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       linux-fsdevel@vger.kernel.org,
       Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [patch 1/2] fs: mnt_want_write speedup
Message-ID: <20090402184305.GI28946@ZenIV.linux.org.uk>
References: <20090310143718.GB15977@wotan.suse.de> <1236809477.30142.83.camel@nimitz> <20090312041334.GB1893@wotan.suse.de> <1237403623.8286.196.camel@nimitz> <20090402182210.GB17175@wotan.suse.de>
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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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