linux-kernel - Re: [PATCH v3] mm: thp: Set the accessed flag for old pages on access fault.

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Message-ID: <20121026090715.GB20914@mudshark.cambridge.arm.com>
Date:	Fri, 26 Oct 2012 10:07:15 +0100
From:	Will Deacon <will.deacon@....com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
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>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH v3] mm: thp: Set the accessed flag for old pages on
 access fault.

On Fri, Oct 26, 2012 at 08:44:35AM +0100, Kirill A. Shutemov wrote:
> On Thu, Oct 25, 2012 at 05:44:31PM +0100, Will Deacon 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.
> > 
> > Cc: Chris Metcalf <cmetcalf@...era.com>
> > Cc: Kirill A. Shutemov <kirill@...temov.name>
> > Cc: Andrea Arcangeli <aarcange@...hat.com>
> > Signed-off-by: Will Deacon <will.deacon@....com>
> > ---
> > 
> > Ok chaps, I rebased this thing onto today's next (which basically
> > necessitated a rewrite) so I've reluctantly dropped my acks and kindly
> > ask if you could eyeball the new code, especially where the locking is
> > concerned. In the numa code (do_huge_pmd_prot_none), Peter checks again
> > that the page is not splitting, but I can't see why that is required.
> 
> In handle_mm_fault() we check if the pmd is under splitting without
> page_table_lock. It's kind of speculative cheap check. We need to re-check
> if the PMD is really not under splitting after taking page_table_lock.

I appreciate the need to check whether the thing is splitting, but I thought
that the pmd_same(*pmd, orig_pmd) check after taking the page_table_lock
would be sufficient, because we know that the entry hasn't changed and that
it wasn't splitting before we took the lock. This also mirrors the approach
taken by do_huge_pmd_wp_page.

Is there something I'm missing?

Cheers,

Will
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