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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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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