From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f72.google.com (mail-ed1-f72.google.com [209.85.208.72]) by kanga.kvack.org (Postfix) with ESMTP id 0F7EA6B000A for ; Thu, 15 Nov 2018 04:22:57 -0500 (EST) Received: by mail-ed1-f72.google.com with SMTP id z10so4089890edz.15 for ; Thu, 15 Nov 2018 01:22:57 -0800 (PST) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id m25-v6si1064555ejq.24.2018.11.15.01.22.55 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 15 Nov 2018 01:22:55 -0800 (PST) Date: Thu, 15 Nov 2018 10:22:54 +0100 From: Michal Hocko Subject: Re: [RFC PATCH] mm, proc: report PR_SET_THP_DISABLE in proc Message-ID: <20181115092254.GJ23831@dhcp22.suse.cz> References: <20181015150325.GN18839@dhcp22.suse.cz> <20181016104855.GQ18839@dhcp22.suse.cz> <20181017070531.GC18839@dhcp22.suse.cz> <20181018070031.GW18839@dhcp22.suse.cz> <20181114132306.GX23419@dhcp22.suse.cz> <20181115090242.GH23831@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181115090242.GH23831@dhcp22.suse.cz> 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 15-11-18 10:02:42, Michal Hocko wrote: > On Wed 14-11-18 13:41:12, David Rientjes wrote: > > On Wed, 14 Nov 2018, Michal Hocko wrote: > > > > > > > > Do you know of any other userspace except your usecase? Is there > > > > > > anything fundamental that would prevent a proper API adoption for you? > > > > > > > > > > > > > > > > Yes, it would require us to go back in time and build patched binaries. > > > > > > > > I read that as there is a fundamental problem to update existing > > > > binaries. If that is the case then there surely is no way around it > > > > and another sad page in the screwed up APIs book we provide. > > > > > > > > But I was under impression that the SW stack which actually does the > > > > monitoring is under your controll. Moreover I was under impression that > > > > you do not use the current vanilla kernel so there is no need for an > > > > immediate change on your end. It is trivial to come up with a backward > > > > compatible way to check for the new flag (if it is not present then > > > > fallback to vma flags). > > > > > > > > The userspace had a single way to determine if thp had been disabled for a > > specific vma and that was broken with your commit. We have since fixed > > it. Modifying our software stack to start looking for some field > > somewhere else will not help anybody else that this has affected or will > > affect. I'm interested in not breaking userspace, not trying a wait and > > see approach to see if anybody else complains once we start looking for > > some other field. The risk outweighs the reward, it already broke us, and > > I'd prefer not to even open the possibility of breaking anybody else. > > I very much agree on "do not break userspace" part but this is kind of > gray area. VMA flags are a deep internal implementation detail and > nobody should really depend on it for anything important. The original > motivation for introducing it was CRIU where it is kind of > understandable. I would argue they should find a different way but it is > just too late for them. > > For this particular case there was no other bug report except for yours > and if it is possible to fix it on your end then I would really love to > make the a sensible user interface to query the status. If we are going > to change the semantic of the exported flag again then we risk yet > another breakage. > > Therefore I am asking whether changing your particular usecase to a new > interface is possible because that would allow to have a longerm > sensible user interface rather than another kludge which still doesn't > cover all the usecases (e.g. there is no way to reliably query the > madvise status after your patch). Btw. this is essentially the same kind of problem as http://lkml.kernel.org/r/20181002100531.GC4135@quack2.suse.cz where the conclusion was to come up with a saner interface rather than mimic the previous one. -- Michal Hocko SUSE Labs