From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f70.google.com (mail-pa0-f70.google.com [209.85.220.70]) by kanga.kvack.org (Postfix) with ESMTP id 51CC26B0038 for ; Wed, 7 Sep 2016 23:41:49 -0400 (EDT) Received: by mail-pa0-f70.google.com with SMTP id vp2so75508590pab.3 for ; Wed, 07 Sep 2016 20:41:49 -0700 (PDT) Received: from mga14.intel.com (mga14.intel.com. [192.55.52.115]) by mx.google.com with ESMTPS id z1si37681338pab.287.2016.09.07.20.41.48 for (version=TLS1 cipher=AES128-SHA bits=128/128); Wed, 07 Sep 2016 20:41:48 -0700 (PDT) Subject: Re: [PATCH] Fix region lost in /proc/self/smaps References: <1473231111-38058-1-git-send-email-guangrong.xiao@linux.intel.com> <57D04192.5070704@intel.com> From: Xiao Guangrong Message-ID: <8b800d72-9b28-237c-47a6-604d98a40315@linux.intel.com> Date: Thu, 8 Sep 2016 11:36:11 +0800 MIME-Version: 1.0 In-Reply-To: <57D04192.5070704@intel.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Dave Hansen , pbonzini@redhat.com, akpm@linux-foundation.org, mhocko@suse.com, dan.j.williams@intel.com Cc: 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 On 09/08/2016 12:34 AM, Dave Hansen wrote: > On 09/06/2016 11:51 PM, Xiao Guangrong wrote: >> In order to fix this bug, we make 'file->version' indicate the next VMA >> we want to handle > > This new approach makes it more likely that we'll skip a new VMA that > gets inserted in between the read()s. But, I guess that's OK. We don't > exactly claim to be giving super up-to-date data at the time of read(). Yes, I completely agree with you. :) > > With the old code, was there also a case that we could print out the > same virtual address range more than once? It seems like that could > happen if we had a VMA split between two reads. Yes. > > I think this introduces one oddity: if you have a VMA merge between two > reads(), you might get the same virtual address range twice in your > output. This didn't happen before because we would have just skipped > over the area that got merged. > > Take two example VMAs: > > vma-A: (0x1000 -> 0x2000) > vma-B: (0x2000 -> 0x3000) > > read() #1: prints vma-A, sets m->version=0x2000 > > Now, merge A/B to make C: > > vma-C: (0x1000 -> 0x3000) > > read() #2: find_vma(m->version=0x2000), returns vma-C, prints vma-C > > The user will see two VMAs in their output: > > A: 0x1000->0x2000 > C: 0x1000->0x3000 > > Will it confuse them to see the same virtual address range twice? Or is > there something preventing that happening that I'm missing? > You are right. Nothing can prevent it. However, it is not easy to handle the case that the new VMA overlays with the old VMA already got by userspace. I think we have some choices: 1: One way is completely skipping the new VMA region as current kernel code does but i do not think this is good as the later VMAs will be dropped. 2: show the un-overlayed portion of new VMA. In your case, we just show the region (0x2000 -> 0x3000), however, it can not work well if the VMA is a new created region with different attributions. 3: completely show the new VMA as this patch does. Which one do you prefer? Thanks! -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org