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=-2.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 F0253C32771 for ; Sun, 12 Jan 2020 09:55:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8F87221744 for ; Sun, 12 Jan 2020 09:55:51 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=yandex-team.ru header.i=@yandex-team.ru header.b="BRCiRzje" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732555AbgALJzu (ORCPT ); Sun, 12 Jan 2020 04:55:50 -0500 Received: from forwardcorp1j.mail.yandex.net ([5.45.199.163]:36598 "EHLO forwardcorp1j.mail.yandex.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732545AbgALJzu (ORCPT ); Sun, 12 Jan 2020 04:55:50 -0500 Received: from mxbackcorp1j.mail.yandex.net (mxbackcorp1j.mail.yandex.net [IPv6:2a02:6b8:0:1619::162]) by forwardcorp1j.mail.yandex.net (Yandex) with ESMTP id 53B4A2E0DBD; Sun, 12 Jan 2020 12:55:47 +0300 (MSK) Received: from myt4-18a966dbd9be.qloud-c.yandex.net (myt4-18a966dbd9be.qloud-c.yandex.net [2a02:6b8:c00:12ad:0:640:18a9:66db]) by mxbackcorp1j.mail.yandex.net (mxbackcorp/Yandex) with ESMTP id B2PIGpo5Xh-tkW01GBU; Sun, 12 Jan 2020 12:55:47 +0300 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=yandex-team.ru; s=default; t=1578822947; bh=rOLVBONEBziUYshGUMG5uQS0vfmDbQkCc6Vf0mb6A+8=; h=In-Reply-To:Message-ID:From:Date:References:To:Subject:Cc; b=BRCiRzjexyHs0KHl4MAbY9JmB/wcODy4/mire7I8KP5WCLP30tEc3LlNImeluHjDr 2ZR/ftMokaRPIDcWEDk4tWz960yH/oOfs44+izK3nHbJfagbsVdqP8/fMvvzaVwcsO RfDC9rfvLVg7MmRWjNlqaS1y6YdsjUItOGUIB6fs= Authentication-Results: mxbackcorp1j.mail.yandex.net; dkim=pass header.i=@yandex-team.ru Received: from unknown (unknown [2a02:6b8:b080:8305::1:0]) by myt4-18a966dbd9be.qloud-c.yandex.net (smtpcorp/Yandex) with ESMTPSA id 21n1R1spZg-tkVSOajX; Sun, 12 Jan 2020 12:55:46 +0300 (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)) (Client certificate not present) Subject: Re: [PATCH v2 1/2] mm/rmap: fix and simplify reusing mergeable anon_vma as parent when fork To: Wei Yang Cc: Li Xinhai , "linux-mm@kvack.org" , akpm , "linux-kernel@vger.kernel.org" , Rik van Riel , "kirill.shutemov" References: <157839239609.694.10268055713935919822.stgit@buzz> <20200108023211.GC13943@richard> <20200109025240.GA2000@richard> <20200110023029.GB16823@richard> <20200110112357351531132@gmail.com> <20200110053442.GA27846@richard> <20200111223820.GA15506@richard> From: Konstantin Khlebnikov Message-ID: Date: Sun, 12 Jan 2020 12:55:45 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2 MIME-Version: 1.0 In-Reply-To: <20200111223820.GA15506@richard> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-CA Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/01/2020 01.38, Wei Yang wrote: > On Fri, Jan 10, 2020 at 11:11:23AM +0300, Konstantin Khlebnikov wrote: > [...] >>>>>> >>>>>> series of vma in parent with shared AV: >>>>>> >>>>>> SRC1 - AV0 >>>>>> SRC2 - AV0 >>>>>> SRC3 - AV0 >>>>>> ... >>>>>> SRCn - AV0 >>>>>> >>>>>> in child after fork >>>>>> >>>>>> DST1 - AV_OLD_1 (some old vma, picked by anon_vma_clone) plus DST1 is attached to same AVs as SRC1 >>>>>> DST2 - AV_OLD_2 (other old vma) plus DST1 is attached to same AVs as SRC2 >>>>>> DST2 - AV1 prev AV parent does not match AV0, no old vma found for reusing -> allocate new one (child of AV0) >>>>>> DST3 - AV1 - DST2->AV->parent == SRC3->AV (AV0) -> share AV with prev >>>>>> DST4 - AV1 - same thing >>>>>> ... >>>>>> DSTn - AV1 >>>>>> > > To focus on the point, I rearranged the order a little. Suppose your following > comments is explaining the above behavior. > > I've illustrated how two heuristics (reusing-old and sharing-prev) _could_ work together. > But they both are optional. > > At cloning first vma SRC1 -> DST1 there is no prev to share anon vma, > thus works common code which _could_ reuse old vma because it have to. > > If there is no old anon-vma which have to be reused then DST1 will allocate > new anon-vma (AV1) and it will be used by DST2 and so on like on your picture. > > I agree with your 3rd paragraph, but confused with 2nd. > > At cloning first vma SRC1 -> DST1, there is no prev so anon_vma_clone() would > pick up a reusable anon_vma. Here you named it AV_OLD_1. This looks good to > me. But I am not sure why you would picked up AV_OLD_2 for DST2? In parent, > SRC1 and SRC2 has the same anon_vma, AV0. So in child, DST1 and DST2 could > also share the same anon_vma, AV_OLD_1. > > Sorry for my poor understanding, would you mind giving me more hint on this > change? For DST2 heuristic "share-with-prev" will not work because if prev (DST1) uses old AV (AV_OLD_1) and AV_OLD_1->parent isn't SRC2->AV (AV0). So DST2 could only pick another old AV or allocate new. My patch uses condition dst->prev->anon_vma->parent == src->anon_vma rather than obvious src->prev->anon_vma == src->anon_vma because in this way it eliminates all unwanted corner cases and explicitly verifies that we going to share related anon-vma. Heuristic "reuse-old" uses fact that VMA links and AV parent chain are tracked independently: when VMA reuses old AV it still links to all related AV even if VMA->AV points into some old AV in the middle of inheritance chain. > >>>>> >>>>> Yes, your code works for DST3..DSTn. They will pick up AV1 since >>>>> (DST2->AV->parent == SRC3->AV). >>>>> >>>>> My question is why DST1 and DST2 has different AV? The purpose of my patch >>>>> tries to make child has the same topology and parent. So the ideal look of >>>>> child is: >>>>> >>>>> DST1 - AV1 >>>>> DST2 - AV1 >>>>> DST2 - AV1 >>>>> DST3 - AV1 >>>>> DST4 - AV1 >>>>> >>>>> Would you mind putting more words on DST1 and DST2? I didn't fully understand >>>>> the logic here. >>>>> >>>>> Thanks >>>>> >>>> >>>> I think that the first version is doing the work as you expected, but been >>>> revised in second version, to limits the number of users of reused old >>>> anon(which is picked in anon_vma_clone() and keep the tree structure. >>>> >>> >>> Any reason to reduce the reuse? Maybe I lost some point. >> >>> >>>>> -- >>>>> Wei Yang >>>>> Help you, Help me >>> >