From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933267AbbFVLPW (ORCPT ); Mon, 22 Jun 2015 07:15:22 -0400 Received: from mta-out1.inet.fi ([62.71.2.229]:44477 "EHLO johanna4.rokki.sonera.fi" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752765AbbFVLPC (ORCPT ); Mon, 22 Jun 2015 07:15:02 -0400 RazorGate-KAS: Rate: 5 RazorGate-KAS: {RECEIVED: dynamic ip detected} RazorGate-KAS: Envelope from: RazorGate-KAS: Version: 5.5.3 RazorGate-KAS: LuaCore: 215 2015-05-29_17-31-22 60ae4a1b4d01d14f868b20a55aced8d7df7b2e28 RazorGate-KAS: Method: none RazorGate-KAS: Lua profiles 78662 [Jun 02 2015] RazorGate-KAS: Status: not_detected Date: Mon, 22 Jun 2015 14:14:34 +0300 From: "Kirill A. Shutemov" To: Vlastimil Babka Cc: "Kirill A. Shutemov" , Andrew Morton , Andrea Arcangeli , Hugh Dickins , Dave Hansen , Mel Gorman , Rik van Riel , Christoph Lameter , Naoya Horiguchi , Steve Capper , "Aneesh Kumar K.V" , Johannes Weiner , Michal Hocko , Jerome Marchand , Sasha Levin , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCHv6 29/36] thp: implement split_huge_pmd() Message-ID: <20150622111434.GC7934@node.dhcp.inet.fi> References: <1433351167-125878-1-git-send-email-kirill.shutemov@linux.intel.com> <1433351167-125878-30-git-send-email-kirill.shutemov@linux.intel.com> <557959BC.5000303@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <557959BC.5000303@suse.cz> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 11, 2015 at 11:49:48AM +0200, Vlastimil Babka wrote: > 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 > >Tested-by: Sasha Levin > > [...] > > >+ > >+ 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? Good catch. Thanks. What about this? diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 0f1f5731a893..cd0e6addb662 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2636,15 +2636,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, for (i = 0; i < HPAGE_PMD_NR; i++) atomic_dec(&page[i]._mapcount); } - } else if (!TestSetPageDoubleMap(page)) { + } else if (!PageDoubleMap(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. + * + * Note, we need to increment mapcounts before setting + * PG_double_map to avoid false-negative page_mapped(). */ for (i = 0; i < HPAGE_PMD_NR; i++) atomic_inc(&page[i]._mapcount); + + if (TestSetPageDoubleMap(page)) { + /* Race with another __split_huge_pmd() for the page */ + for (i = 0; i < HPAGE_PMD_NR; i++) + atomic_dec(&page[i]._mapcount); + } } smp_wmb(); /* make pte visible before pmd */ > 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. I think it's safe. -- Kirill A. Shutemov -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in Please read the FAQ at http://www.tux.org/lkml/