From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1763255AbcINPjJ (ORCPT ); Wed, 14 Sep 2016 11:39:09 -0400 Received: from mx1.redhat.com ([209.132.183.28]:50292 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754281AbcINPjG (ORCPT ); Wed, 14 Sep 2016 11:39:06 -0400 Date: Wed, 14 Sep 2016 17:38:14 +0200 From: Oleg Nesterov To: Dave Hansen Cc: Michal Hocko , Xiao Guangrong , 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 Subject: Re: [PATCH v2] mm, proc: Fix region lost in /proc/self/smaps Message-ID: <20160914153814.GA21284@redhat.com> 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> <20160913145906.GA28037@redhat.com> <57D8277E.80505@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <57D8277E.80505@intel.com> User-Agent: Mutt/1.5.18 (2008-05-17) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Wed, 14 Sep 2016 15:39:05 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/13, Dave Hansen wrote: > > On 09/13/2016 07:59 AM, Oleg Nesterov wrote: > > I agree. I don't even understand why this was considered as a bug. > > Obviously, m_stop() which drops mmap_sep should not be called, or > > all the threads should be stopped, if you want to trust the result. > > There was a mapping at a given address. That mapping did not change, it > was not split, its attributes did not change. But, it didn't show up > when reading smaps. Folks _actually_ noticed this in a test suite > looking for that address range in smaps. I understand, and I won't argue with any change which makes the things better. Just I do not think this is a real problem. And this patch can't fix other oddities and it seems it adds another one (at least) although I can easily misread this patch and/or the code. So we change m_cache_vma(), - m->version = m_next_vma(m->private, vma) ? vma->vm_start : -1UL; + m->version = m_next_vma(m->private, vma) ? vma->vm_end : -1UL; OK, and another change in m_start() - if (vma && (vma = m_next_vma(priv, vma))) + if (vma) means that it can return the same vma if it grows in between. show_map_vma() has another change + start = max(vma->vm_start, start); so it will be reported as _another_ vma, and this doesn't look exactly right. And after that *ppos will be falsely incremented... but probably this doesn't matter because the "if (pos < mm->map_count)" logic in m_start() looks broken anyway. > IOW, we had goofy kernel behavior, and it broke a reasonable test > program. The test program just used fgets() to read into a fixed-length > buffer, which is a completely normal thing to do. > > To get "sensible results", doesn't userspace have to somehow know in > advance how many bytes of data a given VMA will generate in smaps output? Yes, /proc/has its limitations ;) Even if you read, say, /proc/pid/status you can get the corrupted result after the short read. But in this case fgets() should likely work, yes. Dave, let me repeat, I won't argue with any change and in any case you can safely ignore my opinion. Oleg.