From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f70.google.com (mail-ed1-f70.google.com [209.85.208.70]) by kanga.kvack.org (Postfix) with ESMTP id D6F586B0006 for ; Tue, 9 Oct 2018 04:33:28 -0400 (EDT) Received: by mail-ed1-f70.google.com with SMTP id x44-v6so795633edd.17 for ; Tue, 09 Oct 2018 01:33:28 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id ci1-v6si41646ejb.48.2018.10.09.01.33.27 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 09 Oct 2018 01:33:27 -0700 (PDT) Date: Tue, 9 Oct 2018 10:33:26 +0200 From: Michal Hocko Subject: Re: [RFC PATCH] mm, proc: report PR_SET_THP_DISABLE in proc Message-ID: <20181009083326.GG8528@dhcp22.suse.cz> References: <20180925150406.872aab9f4f945193e5915d69@linux-foundation.org> <20180926060624.GA18685@dhcp22.suse.cz> <20181002112851.GP18290@dhcp22.suse.cz> <20181003073640.GF18290@dhcp22.suse.cz> <20181004055842.GA22173@dhcp22.suse.cz> <20181004094637.GG22173@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: David Rientjes Cc: Andrew Morton , Vlastimil Babka , Alexey Dobriyan , "Kirill A. Shutemov" , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-api@vger.kernel.org On Thu 04-10-18 11:34:11, David Rientjes wrote: > On Thu, 4 Oct 2018, Michal Hocko wrote: > > > > And prior to the offending commit, there were three ways to control thp > > > but two ways to determine if a mapping was eligible for thp based on the > > > implementation detail of one of those ways. > > > > Yes, it is really unfortunate that we have ever allowed to leak such an > > internal stuff like VMA flags to userspace. > > > > Right, I don't like userspace dependencies on VmFlags in smaps myself, but > it's the only way we have available that shows whether a single mapping is > eligible to be backed by thp :/ Which is not the case due to reasons mentioned earlier. It only speaks about madvise status on the VMA. > > > If there are three ways to > > > control thp, userspace is still in the dark wrt which takes precedence > > > over the other: we have PR_SET_THP_DISABLE but globally sysfs has it set > > > to "always", or we have MADV_HUGEPAGE set per smaps but PR_SET_THP_DISABLE > > > shown in /proc/pid/status, etc. > > > > > > Which one is the ultimate authority? > > > > Isn't our documentation good enough? If not then we should document it > > properly. > > > > No, because the offending commit actually changed the precedence itself: > PR_SET_THP_DISABLE used to be honored for future mappings and the commit > changed that for all current mappings. Which is the actual and the full point of the fix as described in the changelog. The original implementation was poor and inconsistent. > So as a result of the commit > itself we would have had to change the documentation and userspace can't > be expected to keep up with yet a fourth variable: kernel version. It > really needs to be simpler, just a per-mapping specifier. As I've said, if you really need a per-vma granularity then make it a dedicated line in the output with a clear semantic. Do not make VMA flags even more confusing. -- Michal Hocko SUSE Labs