From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752647AbeDQUwm (ORCPT ); Tue, 17 Apr 2018 16:52:42 -0400 Received: from out30-130.freemail.mail.aliyun.com ([115.124.30.130]:41661 "EHLO out30-130.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751878AbeDQUwl (ORCPT ); Tue, 17 Apr 2018 16:52:41 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R101e4;CH=green;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e07488;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=8;SR=0;TI=SMTPD_---0T.N1GZL_1523998342; Subject: Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct To: Andrew Morton Cc: adobriyan@gmail.com, mhocko@kernel.org, willy@infradead.org, mguzik@redhat.com, gorcunov@gmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <1523730291-109696-1-git-send-email-yang.shi@linux.alibaba.com> <20180417112957.84de526138f404a04298ec4c@linux-foundation.org> From: Yang Shi Message-ID: Date: Tue, 17 Apr 2018 13:52:21 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <20180417112957.84de526138f404a04298ec4c@linux-foundation.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/17/18 11:29 AM, Andrew Morton wrote: > On Sun, 15 Apr 2018 02:24:51 +0800 Yang Shi wrote: > >> mmap_sem is on the hot path of kernel, and it very contended, but it is >> abused too. It is used to protect arg_start|end and evn_start|end when >> reading /proc/$PID/cmdline and /proc/$PID/environ, but it doesn't make >> sense since those proc files just expect to read 4 values atomically and >> not related to VM, they could be set to arbitrary values by C/R. >> >> And, the mmap_sem contention may cause unexpected issue like below: >> >> INFO: task ps:14018 blocked for more than 120 seconds. >> Tainted: G E 4.9.79-009.ali3000.alios7.x86_64 #1 >> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this >> message. >> ps D 0 14018 1 0x00000004 >> ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0 >> ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040 >> 00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000 >> Call Trace: >> [] ? __schedule+0x250/0x730 >> [] schedule+0x36/0x80 >> [] rwsem_down_read_failed+0xf0/0x150 >> [] call_rwsem_down_read_failed+0x18/0x30 >> [] down_read+0x20/0x40 >> [] proc_pid_cmdline_read+0xd9/0x4e0 >> [] ? do_filp_open+0xa5/0x100 >> [] __vfs_read+0x37/0x150 >> [] ? security_file_permission+0x9b/0xc0 >> [] vfs_read+0x96/0x130 >> [] SyS_read+0x55/0xc0 >> [] entry_SYSCALL_64_fastpath+0x1a/0xc5 >> >> Both Alexey Dobriyan and Michal Hocko suggested to use dedicated lock >> for them to mitigate the abuse of mmap_sem. >> >> So, introduce a new spinlock in mm_struct to protect the concurrent >> access to arg_start|end, env_start|end and others, as well as replace >> write map_sem to read to protect the race condition between prctl and >> sys_brk which might break check_data_rlimit(), and makes prctl more >> friendly to other VM operations. > (We should move check_data_rlimit() out of the .h file) I don't get the point, check_data_rlimit() is still used by both prctl and sys_brk. > > It seems inconsistent to be using mmap_sem to protect ->start_brk and > friends in sys_brk(). We've already declared that these are protected > by arg_lock so that's what we should be using? And getting this > consistent should permit us to stop using mmap_sem in prctl() > altogether? Cyrill already helped to elaborate the reason. arg_lock just can protect the concurrent access of brk between prctl calls, but not sys_brk. Thanks, Yang