From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S941383AbcIHOFy (ORCPT ); Thu, 8 Sep 2016 10:05:54 -0400 Received: from mga01.intel.com ([192.55.52.88]:14237 "EHLO mga01.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934412AbcIHOFx (ORCPT ); Thu, 8 Sep 2016 10:05:53 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.30,301,1470726000"; d="scan'208";a="1037065993" Subject: Re: [PATCH] Fix region lost in /proc/self/smaps To: Xiao Guangrong , pbonzini@redhat.com, akpm@linux-foundation.org, mhocko@suse.com, dan.j.williams@intel.com References: <1473231111-38058-1-git-send-email-guangrong.xiao@linux.intel.com> <57D04192.5070704@intel.com> <8b800d72-9b28-237c-47a6-604d98a40315@linux.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 From: Dave Hansen Message-ID: <57D1703E.4070504@intel.com> Date: Thu, 8 Sep 2016 07:05:50 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.8.0 MIME-Version: 1.0 In-Reply-To: <8b800d72-9b28-237c-47a6-604d98a40315@linux.intel.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/07/2016 08:36 PM, Xiao Guangrong wrote:>> 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? I'd be willing to bet that #3 will break *somebody's* tooling. Addresses going backwards is certainly screwy. Imagine somebody using smaps to search for address holes and doing hole_size=0x1000-0x2000. #1 can lies about there being no mapping in place where there there may have _always_ been a mapping and is very similar to the bug you were originally fixing. I think that throws it out. #2 is our best bet, I think. It's unfortunately also the most code. It's also a bit of a fib because it'll show a mapping that never actually existed, but I think this is OK. I'm not sure what the downside is that you're referring to, though. Can you explain?