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

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Date:	Tue, 23 Oct 2012 11:11:25 +0100
From:	Will Deacon <will.deacon@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	David Miller <davem@...emloft.net>,
	"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>,
	"aarcange@...hat.com" <aarcange@...hat.com>,
	"cmetcalf@...era.com" <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 Mon, Oct 22, 2012 at 07:18:43PM +0100, Andrew Morton wrote:
> On Mon, 22 Oct 2012 11:35:03 +0100
> Will Deacon <will.deacon@....com> wrote:
> 
> > On Fri, Oct 19, 2012 at 07:49:55PM +0100, Andrew Morton wrote:
> > > Ho hum.  I'll drop
> > > mm-thp-set-the-accessed-flag-for-old-pages-on-access-fault.patch and
> > > shall assume that you'll sort things out at the appropriate time.
> > 
> > Happy to sort it out once I work out what's going wrong!
> 
> The patch "ARM: mm: Transparent huge page support for LPAE systems" is
> not present in linux-next, so this patch ("mm: thp: Set the accessed
> flag for old pages on access fault") will not compile?

This patch ("mm: thp: Set the accessed flag for old pages on access fault")
doesn't depend on "ARM: mm: Transparent huge page support for LPAE systems"
because currently transparent huge pages cannot be enabled for ARM in
mainline (or linux-next). update_mmu_cache_pmd is only called from
mm/huge_memory.c, which depends on CONFIG_TRANSPARENT_HUGEPAGE=y.

As for the new huge_pmd_set_accessed function... there's a similar situation
for the do_huge_pmd_wp_page function: it's called from mm/memory.c but is
only defined in mm/huge_memory.c. Looks like the compiler optimises those
calls away because pmd_trans_huge and friends constant-fold to 0.

Will
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