Date: Fri, 15 May 2015 14:21:13 +0300
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Hugh Dickins <hughd@google.com>, Dave Hansen <dave.hansen@intel.com>,
        Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>,
        Christoph Lameter <cl@gentwo.org>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Steve Capper <steve.capper@linaro.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
        Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@suse.cz>,
        Jerome Marchand <jmarchan@redhat.com>,
        Sasha Levin <sasha.levin@oracle.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCHv5 04/28] mm, thp: adjust conditions when we can reuse the
 page on WP fault
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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@linux.intel.com>
> >Tested-by: Sasha Levin <sasha.levin@oracle.com>
> 
> Acked-by: Vlastimil Babka <vbabka@suse.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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