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[209.132.180.67]) by mx.google.com with ESMTP id b4si402025qka.207.2020.04.09.17.05.00; Thu, 09 Apr 2020 17:05:15 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=alibaba.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726951AbgDJADG (ORCPT + 99 others); Thu, 9 Apr 2020 20:03:06 -0400 Received: from out30-132.freemail.mail.aliyun.com ([115.124.30.132]:32918 "EHLO out30-132.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726327AbgDJADG (ORCPT ); Thu, 9 Apr 2020 20:03:06 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R381e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01f04397;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=7;SR=0;TI=SMTPD_---0Tv4dqDG_1586476980; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0Tv4dqDG_1586476980) by smtp.aliyun-inc.com(127.0.0.1); Fri, 10 Apr 2020 08:03:03 +0800 Subject: Re: [PATCHv2 5/8] khugepaged: Allow to callapse a page shared across fork From: Yang Shi To: "Kirill A. Shutemov" Cc: akpm@linux-foundation.org, Andrea Arcangeli , Zi Yan , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kirill A. Shutemov" References: <20200403112928.19742-1-kirill.shutemov@linux.intel.com> <20200403112928.19742-6-kirill.shutemov@linux.intel.com> <20200408131044.xzlheacvslrbwrja@box> <107630f5-bbde-3f78-23e9-6f6b3113d709@linux.alibaba.com> Message-ID: <61569ead-2bbd-6d18-c04c-3251218a3fcd@linux.alibaba.com> Date: Thu, 9 Apr 2020 17:03:00 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <107630f5-bbde-3f78-23e9-6f6b3113d709@linux.alibaba.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/8/20 11:51 AM, Yang Shi wrote: > > > On 4/8/20 6:10 AM, Kirill A. Shutemov wrote: >> On Mon, Apr 06, 2020 at 01:50:56PM -0700, Yang Shi wrote: >>> >>> On 4/3/20 4:29 AM, Kirill A. Shutemov wrote: >>>> The page can be included into collapse as long as it doesn't have >>>> extra >>>> pins (from GUP or otherwise). >>>> >>>> Signed-off-by: Kirill A. Shutemov >>>> --- >>>>    mm/khugepaged.c | 25 ++++++++++++++----------- >>>>    1 file changed, 14 insertions(+), 11 deletions(-) >>>> >>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>>> index 57ff287caf6b..1e7e6543ebca 100644 >>>> --- a/mm/khugepaged.c >>>> +++ b/mm/khugepaged.c >>>> @@ -581,11 +581,18 @@ static int >>>> __collapse_huge_page_isolate(struct vm_area_struct *vma, >>>>            } >>>>            /* >>>> -         * cannot use mapcount: can't collapse if there's a gup pin. >>>> -         * The page must only be referenced by the scanned process >>>> -         * and page swap cache. >>>> +         * Check if the page has any GUP (or other external) pins. >>>> +         * >>>> +         * The page table that maps the page has been already >>>> unlinked >>>> +         * from the page table tree and this process cannot get >>>> +         * additinal pin on the page. >>>> +         * >>>> +         * New pins can come later if the page is shared across fork, >>>> +         * but not for the this process. It is fine. The other >>>> process >>>> +         * cannot write to the page, only trigger CoW. >>>>             */ >>>> -        if (page_count(page) != 1 + PageSwapCache(page)) { >>>> +        if (total_mapcount(page) + PageSwapCache(page) != >>>> +                page_count(page)) { >>> This check looks fine for base page, but what if the page is >>> PTE-mapped THP? >>> The following patch made this possible. >>> >>> If it is PTE-mapped THP and the page is in swap cache, the refcount >>> would be >>> 512 + the number of PTE-mapped pages. >>> >>> Shall we do the below change in the following patch? >>> >>> extra_pins = PageSwapCache(page) ? nr_ccompound(page) - 1 : 0; >>> if (total_mapcount(page) + PageSwapCache(page) != page_count(page) - >>> extra_pins) { >>> ... >> Looks like you're right. >> >> It would be nice to have a test case to demonstrate the issue. >> >> Is there any way to trigger moving the page to swap cache? I don't >> see it >> immediately. > > It sounds not easy to trigger since it totally depends on timing, I'm > wondering we may have to use MADV_PAGEOUT? Something below off the top > of my head may trigger this? > > >     CPU       A                                    CPU    B         >             CPU    C > In parent: > MADV_HUGEPAGE > page fault to fill with THP > fork >                                                     In Child: > MADV_NOHUGEPAGE > MADV_DONTNEED (split pmd) > MADV_PAGEOUT >                                                             -> > add_to_swap >                                 khugepaged scan parent and try to > collapse PTE-mapped > >                                                             -> > try_to_unmap > > When doing MADV_DONTNEED we need make sure head page is unmapped since > MADV_PAGEOUT would call page_mapcount(page) to skip shared mapping. This can't trigger it since MADV_PAGEOUT would isolate the page from lru so khugepaged_scan_pmd() can't reach the refcount check at all. If try_to_unmap() or pageout() fails, the page may stay in swap cache. But I didn't think of an easy way to make this happen. > >> >