linux-kernel - Re: [PATCHv6 29/36] thp: implement split_huge

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Message-ID: <557959BC.5000303@suse.cz>
Date:	Thu, 11 Jun 2015 11:49:48 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>
CC:	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: [PATCHv6 29/36] thp: implement split_huge_pmd()

On 06/03/2015 07:06 PM, Kirill A. Shutemov wrote:
> Original split_huge_page() combined two operations: splitting PMDs into
> tables of PTEs and splitting underlying compound page. This patch
> implements split_huge_pmd() which split given PMD without splitting
> other PMDs this page mapped with or underlying compound page.
>
> Without tail page refcounting, implementation of split_huge_pmd() is
> pretty straight-forward.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Tested-by: Sasha Levin <sasha.levin@...cle.com>

[...]

> +
> +	if (atomic_add_negative(-1, compound_mapcount_ptr(page))) {
> +		/* Last compound_mapcount is gone. */
> +		__dec_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
> +		if (PageDoubleMap(page)) {
> +			/* No need in mapcount reference anymore */
> +			ClearPageDoubleMap(page);
> +			for (i = 0; i < HPAGE_PMD_NR; i++)
> +				atomic_dec(&page[i]._mapcount);
> +		}
> +	} else if (!TestSetPageDoubleMap(page)) {
> +		/*
> +		 * The first PMD split for the compound page and we still
> +		 * have other PMD mapping of the page: bump _mapcount in
> +		 * every small page.
> +		 * This reference will go away with last compound_mapcount.
> +		 */
> +		for (i = 0; i < HPAGE_PMD_NR; i++)
> +			atomic_inc(&page[i]._mapcount);

The order of actions here means that between TestSetPageDoubleMap() and 
the atomic incs, anyone calling page_mapcount() on one of the pages not 
processed by the for loop yet, will see a value lower by 1 from what he 
should see. I wonder if that can cause any trouble somewhere, especially 
if there's only one other compound mapping and page_mapcount() will 
return 0 instead of 1?

Conversely, when clearing PageDoubleMap() above (or in one of those rmap 
functions IIRC), one could see mapcount inflated by one. But I guess 
that's less dangerous.

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