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From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Subject: Re: [PATCHv12 26/37] mm: rework mapcount accounting to enable 4k
 mapping of THPs
Date: Fri, 30 Oct 2015 08:33:48 +0000
Message-ID: <20151030083347.GA8259@hori1.linux.bs1.fc.nec.co.jp>
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To: "Kirill A. Shutemov" <kirill@shutemov.name>
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>, Vlastimil Babka <vbabka@suse.cz>, Christoph Lameter <cl@gentwo.org>, 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-kernel@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>

On Thu, Oct 29, 2015 at 11:50:47PM +0200, Kirill A. Shutemov wrote:
...
>=20
> Okay, the problem is that the page was freed under stable_page_flags().
>=20
> Is the code performance sensitive? Can we get reference to the page befor=
e
> touching it? If not, we can rewrite the helper like this:
>
> static inline int PageDoubleMap(struct page *page)
> {
> 	return PageHead(page) && test_bit(PG_double_map, &page[1].flags);       =
                 =20
> }
>=20
> Just dropping the check would be wrong, I think, as we access the next
> page.
>=20

I don't think this interface is performance sensitive, but hopefully the
impact on other workload is minimum. So I like the above rewritten one.

Thanks,
Naoya Horiguchi=

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