linux-kernel - Re: [PATCHv5 19/28] mm: store mapcount for compound page separately

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Date:	Tue, 19 May 2015 11:01:39 +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 19/28] mm: store mapcount for compound page separately

On 05/19/2015 05:55 AM, Kirill A. Shutemov wrote:
>>
>>> +	if (compound_mapcount(page))
>>> +	       ret += compound_mapcount(page) - 1;
>>
>> This looks like it could uselessly duplicate-inline the code for
>> compound_mapcount(). It has atomics and smp_rmb() so I'm not sure if the
>> compiler can just "squash it".
>
> Good point. I'll rework this.

Hm BTW I think same duplication of compound_head() happens in 
lock_page(), where it's done by trylock_page() and then __lock_page(), 
which is also in different compilation unit to make things worse.

I can imagine it's solvable by introducing variants of __lock_page* that 
expect to be already given a head page... if it's worth the trouble.

>>
>> On the other hand, a simple atomic read that was page_mapcount() has turned
>> into multiple atomic reads and flag checks. What about the stability of the
>> whole result? Are all callers ok? (maybe a later page deals with it).
>
> Urghh.. I'll look into this.
>

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