From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754789AbcIMDG6 (ORCPT ); Mon, 12 Sep 2016 23:06:58 -0400 Received: from mga03.intel.com ([134.134.136.65]:19308 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750953AbcIMDG5 (ORCPT ); Mon, 12 Sep 2016 23:06:57 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.30,326,1470726000"; d="scan'208";a="7827718" Subject: Re: [PATCH v2] mm, proc: Fix region lost in /proc/self/smaps To: Michal Hocko , Dave Hansen References: <1473649964-20191-1-git-send-email-guangrong.xiao@linux.intel.com> <20160912125447.GM14524@dhcp22.suse.cz> <57D6C332.4000409@intel.com> <20160912191035.GD14997@dhcp22.suse.cz> Cc: pbonzini@redhat.com, akpm@linux-foundation.org, dan.j.williams@intel.com, gleb@kernel.org, mtosatti@redhat.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, stefanha@redhat.com, yuhuang@redhat.com, linux-mm@kvack.org, ross.zwisler@linux.intel.com, Oleg Nesterov From: Xiao Guangrong Message-ID: Date: Tue, 13 Sep 2016 11:01:09 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0 MIME-Version: 1.0 In-Reply-To: <20160912191035.GD14997@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/13/2016 03:10 AM, Michal Hocko wrote: > On Mon 12-09-16 08:01:06, Dave Hansen wrote: >> On 09/12/2016 05:54 AM, Michal Hocko wrote: >>>>> In order to fix this bug, we make 'file->version' indicate the end address >>>>> of current VMA >>> Doesn't this open doors to another weird cases. Say B would be partially >>> unmapped (tail of the VMA would get unmapped and reused for a new VMA. >> >> In the end, this interface isn't about VMAs. It's about addresses, and >> we need to make sure that the _addresses_ coming out of it are sane. In >> the case that a VMA was partially unmapped, it doesn't make sense to >> show the "new" VMA because we already had some output covering the >> address of the "new" VMA from the old one. > > OK, that is a fair point and it speaks for caching the vm_end rather > than vm_start+skip. > >>> I am not sure we provide any guarantee when there are more read >>> syscalls. Hmm, even with a single read() we can get inconsistent results >>> from different threads without any user space synchronization. >> >> Yeah, very true. But, I think we _can_ at least provide the following >> guarantees (among others): >> 1. addresses don't go backwards >> 2. If there is something at a given vaddr during the entirety of the >> life of the smaps walk, we will produce some output for it. > > I guess we also want > 3. no overlaps with previously printed values (assuming two subsequent > reads without seek). > > the patch tries to achieve the last part as well AFAICS but I guess this > is incomplete because at least /proc//smaps will report counters > for the full vma range while the header (aka show_map_vma) will report > shorter (non-overlapping) range. I haven't checked other files which use > m_{start,next} You are right. Will fix both /proc/PID/smaps and /proc/PID/maps in the next version. > > Considering how this all can be tricky and how partial reads can be > confusing and even misleading I am really wondering whether we > should simply document that only full reads will provide a sensible > results. Make sense. Will document the guarantee in Documentation/filesystems/proc.txt Thank you, Dave and Michal, for figuring out the right direction. :)