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

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Message-ID: <20150515112113.GD6250@node.dhcp.inet.fi>
Date:	Fri, 15 May 2015 14:21:13 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Vlastimil Babka <vbabka@...e.cz>
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 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?

-- 
 Kirill A. Shutemov
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