From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751274AbeBUT50 (ORCPT ); Wed, 21 Feb 2018 14:57:26 -0500 Received: from mail-wr0-f194.google.com ([209.85.128.194]:34722 "EHLO mail-wr0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750716AbeBUT5Y (ORCPT ); Wed, 21 Feb 2018 14:57:24 -0500 X-Google-Smtp-Source: AH8x226xCyn0ddi5Xgvfc6/iVmvktX6UV+Hacr1figdNsBcM8mF/PzQ3P+lnr5g2iW3GvFOC1KndwA== Date: Wed, 21 Feb 2018 22:57:20 +0300 From: Alexey Dobriyan To: Yang Shi Cc: akpm@linux-foundation.org, mingo@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] fs: proc: use down_read_killable in proc_pid_cmdline_read() Message-ID: <20180221195720.GA639@avx2> References: <1519156169-11883-1-git-send-email-yang.shi@linux.alibaba.com> <20180220223825.GA6705@avx2> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.7.2 (2016-11-26) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 20, 2018 at 03:38:24PM -0800, Yang Shi wrote: > > > On 2/20/18 2:38 PM, Alexey Dobriyan wrote: > > On Wed, Feb 21, 2018 at 03:49:29AM +0800, Yang Shi wrote: > >> When running vm-scalability with large memory (> 300GB), the below hung > >> task issue happens occasionally. > >> > >> 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 > >> > >> When manipulating a large mapping, the process may hold the mmap_sem for > >> long time, so reading /proc//cmdline may be blocked in > >> uninterruptible state for long time. > >> > >> down_read_trylock() sounds too aggressive, and we already have killable > >> version APIs for semaphore, here use down_read_killable() to improve the > >> responsiveness. > >> - down_read(&mm->mmap_sem); > >> + rv = down_read_killable(&mm->mmap_sem); > >> + if (rv) > >> + goto out_mmput; > >> arg_start = mm->arg_start; > >> arg_end = mm->arg_end; > >> env_start = mm->env_start; > > Fix is incomplete > > Yes, it is. Since I just ran into the above splat, so I just did the > minimum change. > > > 1) /proc/*/cmdline only wants to read 4 values atomically, > > those 4 values are basically random values and aren't > > related to VM part at all (after C/R went in, they are > > settable to arbitrary values) > > Sorry, I don't get your point here. Could you please elaborate? I hoped there is some random spinlock those 4 values could be moved to but no. > > 2) access_remote_vm() et al will do the same ->mmap_sem, and > > Yes, it does. But, __access_remote_vm() is called by access_process_vm() > too, which is used by much more places, i.e. ptrace, so I was not sure > if it is preferred to convert to killable version. So, I leave it untouched. Yeah, but ->mmap_sem is taken 3 times per /proc/*/cmdline read and your scalability tests should trigger next backtrace right away. > > 3) /proc/*/environ and get_cmdline() do the same. > > They look suitable to use killable version. > > BTW, I just realized the code should go to out_free_page instead of > out_mmput. Will rectify in newer version once we decide the extra places > to use killable version.