linux-kernel - Re: [PATCHv5 04/28] mm, thp: adjust conditions when we can reuse the page on WP fault

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Date:	Fri, 15 May 2015 13:35:49 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
CC:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...two.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Steve Capper <steve.capper@...aro.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Jerome Marchand <jmarchan@...hat.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv5 04/28] mm, thp: adjust conditions when we can reuse
 the page on WP fault

On 05/15/2015 01:21 PM, Kirill A. Shutemov wrote:
> On Fri, May 15, 2015 at 11:15:00AM +0200, Vlastimil Babka wrote:
>> On 04/23/2015 11:03 PM, Kirill A. Shutemov wrote:
>>> With new refcounting we will be able map the same compound page with
>>> PTEs and PMDs. It requires adjustment to conditions when we can reuse
>>> the page on write-protection fault.
>>>
>>> For PTE fault we can't reuse the page if it's part of huge page.
>>>
>>> For PMD we can only reuse the page if nobody else maps the huge page or
>>> it's part. We can do it by checking page_mapcount() on each sub-page,
>>> but it's expensive.
>>>
>>> The cheaper way is to check page_count() to be equal 1: every mapcount
>>> takes page reference, so this way we can guarantee, that the PMD is the
>>> only mapping.
>>>
>>> This approach can give false negative if somebody pinned the page, but
>>> that doesn't affect correctness.
>>>
>>> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
>>> Tested-by: Sasha Levin <sasha.levin@...cle.com>
>>
>> Acked-by: Vlastimil Babka <vbabka@...e.cz>
>>
>> So couldn't the same trick be used in Patch 1 to avoid counting individual
>> oder-0 pages?
>
> Hm. You're right, we could. But is smaps that performance sensitive to
> bother?

Well, I was nudged to optimize it when doing the shmem swap accounting 
changes there :) User may not care about the latency of obtaining the 
smaps file contents, but since it has mmap_sem locked for that, the 
process might care...


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