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

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Date:	Fri, 26 Oct 2012 17:49:08 +0800
From:	Ni zhan Chen <nizhan.chen@...il.com>
To:	Will Deacon <will.deacon@....com>
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>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH v3] mm: thp: Set the accessed flag for old pages on access
 fault.

On 10/26/2012 05:34 PM, Will Deacon wrote:
> On Fri, Oct 26, 2012 at 07:19:55AM +0100, Ni zhan Chen wrote:
>> On 10/26/2012 12:44 AM, 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.
>> Could you write changlog?
> >From v2? I included something below my SoB. The code should do exactly the
> same as before, it's just rebased onto next so that I can play nicely with
> Peter's patches.
>
>>> 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.
>>>
>>> Cheers,
>>>
>>> Will
>> Could you explain why you not call pmd_trans_huge_lock to confirm the
>> pmd is splitting or stable as Andrea point out?
> The way handle_mm_fault is now structured after the numa changes means that
> we only enter the huge pmd page aging code if the entry wasn't splitting

Why you call it huge pmd page *aging* code?

Regards,
Chen

> before taking the lock, so it seemed a bit gratuitous to jump through those
> hoops again in pmd_trans_huge_lock.
>
> Will
>

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