linux-kernel - Re: [PATCH v2] mm: thp: Set the accessed flag for old pages on access fault.

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Message-ID: <20121003090914.GA22445@mudshark.cambridge.arm.com>
Date:	Wed, 3 Oct 2012 10:09:14 +0100
From:	Will Deacon <will.deacon@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"mhocko@...e.cz" <mhocko@...e.cz>,
	"kirill@...temov.name" <kirill@...temov.name>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Steve Capper <Steve.Capper@....com>
Subject: Re: [PATCH v2] mm: thp: Set the accessed flag for old pages on
 access fault.

On Tue, Oct 02, 2012 at 11:01:04PM +0100, Andrew Morton wrote:
> On Tue,  2 Oct 2012 17:59:11 +0100
> Will Deacon <will.deacon@....com> wrote:
> 
> > On x86 memory accesses to pages without the ACCESSED flag set result in the
> > ACCESSED flag being set automatically. With the ARM architecture a page access
> > fault is raised instead (and it will continue to be raised until the ACCESSED
> > flag is set for the appropriate PTE/PMD).
> > 
> > For normal memory pages, handle_pte_fault will call pte_mkyoung (effectively
> > setting the ACCESSED flag). For transparent huge pages, pmd_mkyoung will only
> > be called for a write fault.
> > 
> > This patch ensures that faults on transparent hugepages which do not result
> > in a CoW update the access flags for the faulting pmd.
> 
> Alas, the code you're altering has changed so much in linux-next that I
> am reluctant to force this fix in there myself.  Can you please
> redo/retest/resend?  You can do that on 3.7-rc1 if you like, then we
> can feed this into -rc2.

No problem. I'll rebase the entire ARM series at -rc1 prior to posting
anyway, so this can be included in that lot.

> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3524,7 +3524,8 @@ retry:
> >  
> >  		barrier();
> >  		if (pmd_trans_huge(orig_pmd)) {
> > -			if (flags & FAULT_FLAG_WRITE &&
> > +			int dirty = flags & FAULT_FLAG_WRITE;
> 
> `flags' is `unsigned int', so making `dirty' match that is nicer.

I'll fold that in with the above.

Cheers,

Will
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