From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752707AbeDQUjX (ORCPT ); Tue, 17 Apr 2018 16:39:23 -0400 Received: from mail-lf0-f66.google.com ([209.85.215.66]:41704 "EHLO mail-lf0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752254AbeDQUjW (ORCPT ); Tue, 17 Apr 2018 16:39:22 -0400 X-Google-Smtp-Source: AIpwx49mKZQ+8+2WaWi+N2CJ9sEn540wFojXQYHOP/1HJ0y7pdDwLzcIaHV3QEOkXzlG/in0wcMlLQ== Date: Tue, 17 Apr 2018 23:39:19 +0300 From: Cyrill Gorcunov To: Andrew Morton Cc: Yang Shi , adobriyan@gmail.com, mhocko@kernel.org, willy@infradead.org, mguzik@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct Message-ID: <20180417203919.GF19578@uranus.lan> References: <1523730291-109696-1-git-send-email-yang.shi@linux.alibaba.com> <20180417112957.84de526138f404a04298ec4c@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180417112957.84de526138f404a04298ec4c@linux-foundation.org> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 17, 2018 at 11:29:57AM -0700, 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) > > 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? Nope, we still can't. Look, the down_read part order the call with sys_brk. while arg_lock orders prctl call itself. That said if someone is calling sys_brk while we're in a middle of prctl it should wait until prctl finished. But two simultaneous prcl may proceed without taking a write lock using arg_lock as a barrier.