linux-kernel - Re: [PATCHv12 26/37] mm: rework mapcount accounting to enable 4k mapping of THPs

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Date:	Fri, 30 Oct 2015 08:33:48 +0000
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
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>,
	Vlastimil Babka <vbabka@...e.cz>,
	Christoph Lameter <cl@...two.org>,
	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-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCHv12 26/37] mm: rework mapcount accounting to enable 4k
 mapping of THPs

On Thu, Oct 29, 2015 at 11:50:47PM +0200, Kirill A. Shutemov wrote:
...
> 
> Okay, the problem is that the page was freed under stable_page_flags().
> 
> Is the code performance sensitive? Can we get reference to the page before
> 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);                         
> }
> 
> Just dropping the check would be wrong, I think, as we access the next
> page.
> 

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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