From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933226AbeBWUIo (ORCPT ); Fri, 23 Feb 2018 15:08:44 -0500 Received: from out30-130.freemail.mail.aliyun.com ([115.124.30.130]:42113 "EHLO out30-130.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932898AbeBWUIm (ORCPT ); Fri, 23 Feb 2018 15:08:42 -0500 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R991e4;CH=green;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01f04446;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=4;SR=0;TI=SMTPD_---0SyOsBP1_1519416504; Subject: Re: [PATCH] fs: proc: use down_read_killable in proc_pid_cmdline_read() To: Alexey Dobriyan Cc: akpm@linux-foundation.org, mingo@kernel.org, linux-kernel@vger.kernel.org References: <1519156169-11883-1-git-send-email-yang.shi@linux.alibaba.com> <20180220223825.GA6705@avx2> <20180221195720.GA639@avx2> <20180223193338.GA5708@avx2> <20180223194547.GC5708@avx2> From: Yang Shi Message-ID: <139a6862-ca1a-c291-3e03-8130e35b5fc0@linux.alibaba.com> Date: Fri, 23 Feb 2018 12:08:20 -0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <20180223194547.GC5708@avx2> 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 2/23/18 11:45 AM, Alexey Dobriyan wrote: > On Fri, Feb 23, 2018 at 11:42:34AM -0800, Yang Shi wrote: >> >> On 2/23/18 11:33 AM, Alexey Dobriyan wrote: >>> On Wed, Feb 21, 2018 at 03:13:10PM -0800, Yang Shi wrote: >>> >>>>>>> 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. >>>> Yes, however, I didn't run into it if mmap_sem is acquired earlier. >>>> >>>> How about defining a killable version, like >>>> __access_remote_vm_killable() which use down_read_killable(), then the >>>> killable version can be used by proc/*/cmdline? There might be other >>>> users in the future. >>> It would be a disaster as interfaces multiply. >> Might be not that bad. > Maybe. > > But you need to explain why there is no backtrace several lines later: > > access_remote_vm > __access_remote_vm > down_read(&mm->mmap_sem) I think it might be because: CPU A CPU B read /proc/*/cmdline get_mm acquire mmap_sem munmap(300G) try to acquire mmap_sem --> go to sleep release mmap_sem got mmap_sem release mmap_sem access_remote_vm put_mm The munmap might happen right before access_remote_vm(), but I just didn't run into it for the time being. It may be hit on another machine or with some changes to the test cases. BTW, even the hung I met happened occassionally, not very often. So, the access_remote_vm() hit sounds less often. But, I agree it is still possible in theory. Regards, Yang