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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 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 A8780C47404 for ; Fri, 11 Oct 2019 05:21:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 80DD12089F for ; Fri, 11 Oct 2019 05:21:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726358AbfJKFV1 (ORCPT ); Fri, 11 Oct 2019 01:21:27 -0400 Received: from mga18.intel.com ([134.134.136.126]:60995 "EHLO mga18.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726099AbfJKFV1 (ORCPT ); Fri, 11 Oct 2019 01:21:27 -0400 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 orsmga106.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 10 Oct 2019 22:21:26 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.67,282,1566889200"; d="scan'208";a="200682314" Received: from richard.sh.intel.com (HELO localhost) ([10.239.159.54]) by FMSMGA003.fm.intel.com with ESMTP; 10 Oct 2019 22:21:24 -0700 Date: Fri, 11 Oct 2019 13:21:07 +0800 From: Wei Yang To: Wei Yang Cc: akpm@linux-foundation.org, kirill.shutemov@linux.intel.com, jglisse@redhat.com, mike.kravetz@oracle.com, riel@surriel.com, khlebnikov@yandex-team.ru, cai@lca.pw, shakeelb@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [Patch v3 1/2] mm/rmap.c: don't reuse anon_vma if we just want a copy Message-ID: <20191011052107.GA22714@richard> Reply-To: Wei Yang References: <20191011025841.16801-1-richardw.yang@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191011025841.16801-1-richardw.yang@linux.intel.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 11, 2019 at 10:58:40AM +0800, Wei Yang wrote: >Before commit 7a3ef208e662 ("mm: prevent endless growth of anon_vma >hierarchy"), anon_vma_clone() doesn't change dst->anon_vma. While after >this commit, anon_vma_clone() will try to reuse an exist one on forking. > >But this commit go a little bit further for the case not forking. >anon_vma_clone() is called from __vma_split(), __split_vma(), copy_vma() >and anon_vma_fork(). For the first three places, the purpose here is get >a copy of src and we don't expect to touch dst->anon_vma even it is >NULL. While after that commit, it is possible to reuse an anon_vma when >dst->anon_vma is NULL. This is not we intend to have. > >This patch stop reuse anon_vma for non-fork cases. > >Fix commit 7a3ef208e662 ("mm: prevent endless growth of anon_vma >hierarchy") > >Signed-off-by: Wei Yang > >--- >v3: > * use dst->anon_vma and src->anon_vma to get reuse state > pointed by Konstantin Khlebnikov >--- > mm/rmap.c | 12 +++++++++--- > 1 file changed, 9 insertions(+), 3 deletions(-) > >diff --git a/mm/rmap.c b/mm/rmap.c >index d9a23bb773bf..fc0aba7fb9b9 100644 >--- a/mm/rmap.c >+++ b/mm/rmap.c >@@ -250,7 +250,13 @@ static inline void unlock_anon_vma_root(struct anon_vma *root) > * Attach the anon_vmas from src to dst. > * Returns 0 on success, -ENOMEM on failure. > * >- * If dst->anon_vma is NULL this function tries to find and reuse existing >+ * anon_vma_clone() is called by __vma_split(), __split_vma(), copy_vma() and >+ * anon_vma_fork(). The first three want an exact copy of src, while the last >+ * one, anon_vma_fork(), may try to reuse an existing anon_vma to prevent >+ * endless growth of anon_vma. Since dst->anon_vma is set to NULL before call, >+ * we can identify this case by (reuse = !dst->anon_vma && src->anon_vma). >+ * >+ * If reuse is true, this function tries to find and reuse existing > * anon_vma which has no vmas and only one child anon_vma. This prevents > * degradation of anon_vma hierarchy to endless linear chain in case of > * constantly forking task. On the other hand, an anon_vma with more than one >@@ -262,6 +268,7 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src) > { > struct anon_vma_chain *avc, *pavc; > struct anon_vma *root = NULL; >+ bool reuse = !dst->anon_vma && src->anon_vma; > > list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) { > struct anon_vma *anon_vma; >@@ -286,8 +293,7 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src) > * will always reuse it. Root anon_vma is never reused: > * it has self-parent reference and at least one child. > */ >- if (!dst->anon_vma && anon_vma != src->anon_vma && >- anon_vma->degree < 2) >+ if (reuse && anon_vma != src->anon_vma && anon_vma->degree < 2) > dst->anon_vma = anon_vma; What a shame. dst->anon_vma would be changed in the loop, so we only need to assign it when dst->anon_vma == NULL. > } > if (dst->anon_vma) >-- >2.17.1 -- Wei Yang Help you, Help me