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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id EDCA9C433EF for ; Tue, 8 Mar 2022 12:09:22 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 6C3D48D0002; Tue, 8 Mar 2022 07:09:22 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 64C778D0001; Tue, 8 Mar 2022 07:09:22 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 4ED688D0002; Tue, 8 Mar 2022 07:09:22 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0238.hostedemail.com [216.40.44.238]) by kanga.kvack.org (Postfix) with ESMTP id 3F54D8D0001 for ; Tue, 8 Mar 2022 07:09:22 -0500 (EST) Received: from smtpin27.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 09FE2A5A75 for ; Tue, 8 Mar 2022 12:09:22 +0000 (UTC) X-FDA: 79221098964.27.1F423B2 Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) by imf21.hostedemail.com (Postfix) with ESMTP id BCAD21C000B for ; Tue, 8 Mar 2022 12:09:20 +0000 (UTC) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.53]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4KCYsT5fmxzbcCZ; Tue, 8 Mar 2022 20:04:29 +0800 (CST) Received: from [10.174.177.76] (10.174.177.76) by canpemm500002.china.huawei.com (7.192.104.244) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2308.21; Tue, 8 Mar 2022 20:09:16 +0800 Subject: Re: [PATCH 04/16] mm/migration: reduce the rcu lock duration To: "Huang, Ying" CC: , , , , , , , , , , , , , , , , , , Christoph Lameter , David Howells , Linus Torvalds References: <20220304093409.25829-1-linmiaohe@huawei.com> <20220304093409.25829-5-linmiaohe@huawei.com> <8735ju7as9.fsf@yhuang6-desk2.ccr.corp.intel.com> From: Miaohe Lin Message-ID: <2eb3fc34-3c81-394f-3bca-8eb00027afcf@huawei.com> Date: Tue, 8 Mar 2022 20:09:15 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <8735ju7as9.fsf@yhuang6-desk2.ccr.corp.intel.com> Content-Type: text/plain; charset="windows-1252" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.177.76] X-ClientProxiedBy: dggems706-chm.china.huawei.com (10.3.19.183) To canpemm500002.china.huawei.com (7.192.104.244) X-CFilter-Loop: Reflected X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: BCAD21C000B X-Stat-Signature: adu4s3jxc69r1yejgo7zi77wxfwp7f6e Authentication-Results: imf21.hostedemail.com; dkim=none; dmarc=pass (policy=quarantine) header.from=huawei.com; spf=pass (imf21.hostedemail.com: domain of linmiaohe@huawei.com designates 45.249.212.187 as permitted sender) smtp.mailfrom=linmiaohe@huawei.com X-Rspam-User: X-HE-Tag: 1646741360-663200 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 2022/3/7 10:32, Huang, Ying wrote: > Miaohe Lin writes: > >> rcu_read_lock is required by grabbing the task refcount but it's not >> needed for ptrace_may_access. So we could release the rcu lock after >> task refcount is successfully grabbed to reduce the rcu holding time. >> >> Signed-off-by: Miaohe Lin >> --- >> mm/migrate.c | 3 +-- >> 1 file changed, 1 insertion(+), 2 deletions(-) >> >> diff --git a/mm/migrate.c b/mm/migrate.c >> index da5a81052468..26943bd819e8 100644 >> --- a/mm/migrate.c >> +++ b/mm/migrate.c >> @@ -1907,17 +1907,16 @@ static struct mm_struct *find_mm_struct(pid_t pid, nodemask_t *mem_nodes) >> return ERR_PTR(-ESRCH); >> } >> get_task_struct(task); >> + rcu_read_unlock(); >> >> /* >> * Check if this process has the right to modify the specified >> * process. Use the regular "ptrace_may_access()" checks. >> */ >> if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) { >> - rcu_read_unlock(); >> mm = ERR_PTR(-EPERM); >> goto out; >> } >> - rcu_read_unlock(); >> >> mm = ERR_PTR(security_task_movememory(task)); >> if (IS_ERR(mm)) > > Digged some history via `git blame`, found that the RCU read lock is > extended in the following commit, > > " > 3268c63eded4612a3d07b56d1e02ce7731e6608e > Author: Christoph Lameter > AuthorDate: Wed Mar 21 16:34:06 2012 -0700 > Commit: Linus Torvalds > CommitDate: Wed Mar 21 17:54:58 2012 -0700 > > mm: fix move/migrate_pages() race on task struct > > Migration functions perform the rcu_read_unlock too early. As a result > the task pointed to may change from under us. This can result in an oops, > as reported by Dave Hansen in https://lkml.org/lkml/2012/2/23/302. > > The following patch extend the period of the rcu_read_lock until after the > permissions checks are done. We also take a refcount so that the task > reference is stable when calling security check functions and performing > cpuset node validation (which takes a mutex). > > The refcount is dropped before actual page migration occurs so there is no > change to the refcounts held during page migration. > > Also move the determination of the mm of the task struct to immediately > before the do_migrate*() calls so that it is clear that we switch from > handling the task during permission checks to the mm for the actual > migration. Since the determination is only done once and we then no > longer use the task_struct we can be sure that we operate on a specific > address space that will not change from under us. > " > > After that, the permission checking has been changed from __task_cred() > to ptrace_may_access(). So the situation may change somewhat. Cced In ptrace_may_access, __task_cred is access while holding the rcu read lock. It seems this is ensured by the ptrace_may_access itself. > some names found in git history to verify. Thanks for your carefulness. > > Best Regards, > Huang, Ying > . >