From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.8 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 08F4DC2BB86 for ; Fri, 10 Apr 2020 15:56:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CB76F20CC7 for ; Fri, 10 Apr 2020 15:56:18 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=shutemov-name.20150623.gappssmtp.com header.i=@shutemov-name.20150623.gappssmtp.com header.b="t2Hk/GfO" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726679AbgDJP4R (ORCPT ); Fri, 10 Apr 2020 11:56:17 -0400 Received: from mail-lf1-f66.google.com ([209.85.167.66]:41391 "EHLO mail-lf1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726080AbgDJP4Q (ORCPT ); Fri, 10 Apr 2020 11:56:16 -0400 Received: by mail-lf1-f66.google.com with SMTP id u10so499394lfo.8 for ; Fri, 10 Apr 2020 08:56:15 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=shutemov-name.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:content-transfer-encoding:in-reply-to; bh=5oXzzweU9+b1N5+cGj6pvbd+Y8KpBQDkz61NLnD/b7c=; b=t2Hk/GfOBLvuuJ342Lrin5iCn8jW7Hdl70kSAp5chiaxVe3jyF6APhVBUz0JD9wR9C csCdUGmml/oC1rstoIwryKphaM37BYtFVKrv+Uz5ib0rljmOqRayvK04I+Ux3bBdZVZU gidlwxUtSxfhwE5i6DHR013lTGAu27xnhQ61ditOHV5ZRlPksyJtrt/KC9KC72u32hLC kNju4qiF4rOQOszUwaVQAvaZHVMM6+RuNrCa79UvgBzMyJnW/j3sAZ75qtAnkjiJi0UK 86ipF3Ny/DWYDMNdjEAFMrqs73wIyoWWxB3kkB9oq5Xhs5ZNcQ/4IMxXsnqzS5Xf1218 gnjA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:content-transfer-encoding :in-reply-to; bh=5oXzzweU9+b1N5+cGj6pvbd+Y8KpBQDkz61NLnD/b7c=; b=HbyuL72orRpxUdim6X8fuTh2T7CxciAgnunAju3y9NL+G1oizeQigKKb8CV7apteJD 58DvpLkwjOS7MGXpp09Y75PUHmyG0s6Ihbb81m4fY8uBCQO2OMS5uCsCinJIEPdgQ4Ef RZmf5PYZGayga6K5za/8p/WUhhtWSqIZH/QagdK12bk6pJfB/hdRmDXkAtn2WFvP4o4l PzOdIki+UaRjbz11QVFnLC7dKYfmSHps2l3uXosk735M0fsmonBIwTWzWP+71Wo1+zRk LiWtt86iJMpty+etfFTVGAj26SwO82ncc4rlxNMZFbjxuimF1z08fL1Ctbmq7HocLrXL Kpmw== X-Gm-Message-State: AGi0PuaIXWXqk1wKO4OyAI4Horb1xAn0Eoo1/t96BZRsWVd1rIsrQc0M K4nENjzNxdkzld0pcy5+kyIkvA== X-Google-Smtp-Source: APiQypI9qvle8foeI1LCtXi2mzT1MhdY9shhQWtO1nZMume9pEjCI8OG2k/lFRxBm7w9FG5W1mkOMw== X-Received: by 2002:a05:6512:686:: with SMTP id t6mr3048293lfe.163.1586534174501; Fri, 10 Apr 2020 08:56:14 -0700 (PDT) Received: from box.localdomain ([86.57.175.117]) by smtp.gmail.com with ESMTPSA id w10sm1264769ljh.85.2020.04.10.08.56.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 10 Apr 2020 08:56:13 -0700 (PDT) Received: by box.localdomain (Postfix, from userid 1000) id DA8EE102110; Fri, 10 Apr 2020 18:56:13 +0300 (+03) Date: Fri, 10 Apr 2020 18:56:13 +0300 From: "Kirill A. Shutemov" To: Yang Shi Cc: akpm@linux-foundation.org, Andrea Arcangeli , Zi Yan , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kirill A. Shutemov" Subject: Re: [PATCHv2 5/8] khugepaged: Allow to callapse a page shared across fork Message-ID: <20200410155613.cxqplrmwckezx3hl@box> 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> <61569ead-2bbd-6d18-c04c-3251218a3fcd@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <61569ead-2bbd-6d18-c04c-3251218a3fcd@linux.alibaba.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 09, 2020 at 05:03:00PM -0700, Yang Shi wrote: > > > 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. Okay, I'll fix it without a test case. -- Kirill A. Shutemov