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=-2.2 required=3.0 tests=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 EDDC7C33CA9 for ; Mon, 13 Jan 2020 00:33:55 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 83B8822314 for ; Mon, 13 Jan 2020 00:33:55 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 83B8822314 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.intel.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id EEAAD8E0005; Sun, 12 Jan 2020 19:33:54 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id E9D0A8E0001; Sun, 12 Jan 2020 19:33:54 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id DB2408E0005; Sun, 12 Jan 2020 19:33:54 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0241.hostedemail.com [216.40.44.241]) by kanga.kvack.org (Postfix) with ESMTP id C7DEA8E0001 for ; Sun, 12 Jan 2020 19:33:54 -0500 (EST) Received: from smtpin30.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with SMTP id 7DA6B824934B for ; Mon, 13 Jan 2020 00:33:54 +0000 (UTC) X-FDA: 76370738388.30.grass04_5c35ef7ce7d42 X-HE-Tag: grass04_5c35ef7ce7d42 X-Filterd-Recvd-Size: 6651 Received: from mga06.intel.com (mga06.intel.com [134.134.136.31]) by imf29.hostedemail.com (Postfix) with ESMTP for ; Mon, 13 Jan 2020 00:33:53 +0000 (UTC) X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga003.fm.intel.com ([10.253.24.29]) by orsmga104.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 12 Jan 2020 16:33:51 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.69,426,1571727600"; d="scan'208";a="272897797" Received: from richard.sh.intel.com (HELO localhost) ([10.239.159.54]) by FMSMGA003.fm.intel.com with ESMTP; 12 Jan 2020 16:33:49 -0800 Date: Mon, 13 Jan 2020 08:33:43 +0800 From: Wei Yang To: Konstantin Khlebnikov Cc: Wei Yang , Li Xinhai , "linux-mm@kvack.org" , akpm , "linux-kernel@vger.kernel.org" , Rik van Riel , "kirill.shutemov" Subject: Re: [PATCH v2 1/2] mm/rmap: fix and simplify reusing mergeable anon_vma as parent when fork Message-ID: <20200113003343.GA27210@richard> Reply-To: Wei Yang References: <20200108023211.GC13943@richard> <20200109025240.GA2000@richard> <20200110023029.GB16823@richard> <20200110112357351531132@gmail.com> <20200110053442.GA27846@richard> <20200111223820.GA15506@richard> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Sun, Jan 12, 2020 at 12:55:45PM +0300, Konstantin Khlebnikov wrote: > > >On 12/01/2020 01.38, Wei Yang wrote: >> On Fri, Jan 10, 2020 at 11:11:23AM +0300, Konstantin Khlebnikov wrote: >> [...] >> > > > > >=20 >> > > > > > series of vma in parent with shared AV: >> > > > > >=20 >> > > > > > SRC1 - AV0 >> > > > > > SRC2 - AV0 >> > > > > > SRC3 - AV0 >> > > > > > ... >> > > > > > SRCn - AV0 >> > > > > >=20 >> > > > > > in child after fork >> > > > > >=20 >> > > > > > DST1 - AV_OLD_1 (some old vma, picked by anon_vma_clone) plu= s DST1 is attached to same AVs as SRC1 >> > > > > > DST2 - AV_OLD_2 (other old vma) plus DST1 is attached to sam= e AVs as SRC2 >> > > > > > DST2 - AV1 prev AV parent does not match AV0, no old vma fou= nd for reusing -> allocate new one (child of AV0) >> > > > > > DST3 - AV1 - DST2->AV->parent =3D=3D SRC3->AV (AV0) -> share= AV with prev >> > > > > > DST4 - AV1 - same thing >> > > > > > ... >> > > > > > DSTn - AV1 >> > > > > >=20 >>=20 >> To focus on the point, I rearranged the order a little. Suppose your f= ollowing >> comments is explaining the above behavior. >>=20 >> 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 v= ma, >> 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 y= our picture. >>=20 >> I agree with your 3rd paragraph, but confused with 2nd. >>=20 >> 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 go= od to >> me. But I am not sure why you would picked up AV_OLD_2 for DST2? In pa= rent, >> SRC1 and SRC2 has the same anon_vma, AV0. So in child, DST1 and DST2 c= ould >> also share the same anon_vma, AV_OLD_1. >>=20 >> 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. I know this behavior after your change, my question is why you want to do= so. > >My patch uses condition dst->prev->anon_vma->parent =3D=3D src->anon_vma= rather >than obvious src->prev->anon_vma =3D=3D src->anon_vma because in this wa= y it >eliminates all unwanted corner cases and explicitly verifies that we goi= ng to >share related anon-vma. > This do eliminates some corner case, but as you showed child and parent d= on't share the same AV topology. To keep the same AV topology is the purpose o= f my commit. I agree you found some bug that previous commit doesn't do it is expected= . But since you change the design a little, I suggest you split this idea to a separate patch so that reviewer and audience in the future could understa= nd your approach clearly. Otherwise audience would be confused and hard to t= rack this change. For example, you describe the behavior after your change. The second vma = would probably have a different AV from first vma. >Heuristic "reuse-old" uses fact that VMA links and AV parent chain are t= racked >independently: when VMA reuses old AV it still links to all related AV e= ven >if VMA->AV points into some old AV in the middle of inheritance chain. > >>=20 >> > > > >=20 >> > > > > Yes, your code works for DST3..DSTn. They will pick up AV1 sin= ce >> > > > > (DST2->AV->parent =3D=3D SRC3->AV). >> > > > >=20 >> > > > > 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 i= deal look of >> > > > > child is: >> > > > >=20 >> > > > > DST1 - AV1 >> > > > > DST2 - AV1 >> > > > > DST2 - AV1 >> > > > > DST3 - AV1 >> > > > > DST4 - AV1 >> > > > >=20 >> > > > > Would you mind putting more words on DST1 and DST2? I didn't f= ully understand >> > > > > the logic here. >> > > > >=20 >> > > > > Thanks >> > > > >=20 >> > > >=20 >> > > > 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 reus= ed old >> > > > anon(which=A0is picked=A0in anon_vma_clone() and keep the tree s= tructure. >> > > >=20 >> > >=20 >> > > Any reason to reduce the reuse? Maybe I lost some point. >> >=20 >> > >=20 >> > > > > -- >> > > > > Wei Yang >> > > > > Help you, Help me >> > >=20 >>=20 --=20 Wei Yang Help you, Help me