Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751486AbdFBVKu (ORCPT ); Fri, 2 Jun 2017 17:10:50 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:42768 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751135AbdFBVKs (ORCPT ); Fri, 2 Jun 2017 17:10:48 -0400 Date: Fri, 2 Jun 2017 14:10:41 -0700 From: Andrew Morton To: Vlastimil Babka Cc: Mike Rapoport , Linux API , Michal Hocko , Andrea Arcangeli , Arnd Bergmann , "Kirill A. Shutemov" , Pavel Emelyanov , linux-mm , lkml , Michal Hocko Subject: Re: [PATCH] mm: make PR_SET_THP_DISABLE immediately active Message-Id: <20170602141041.baace0cfa370b6bec6d411b4@linux-foundation.org> In-Reply-To: References: <1496415802-30944-1-git-send-email-rppt@linux.vnet.ibm.com> <20170602125059.66209870607085b84c257593@linux-foundation.org> <8a810c81-6a72-2af0-a450-6f03c71d8cca@suse.cz> <20170602134038.13728cb77678ae1a7d7128a4@linux-foundation.org> X-Mailer: Sylpheed 3.4.1 (GTK+ 2.24.23; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1910 Lines: 49 On Fri, 2 Jun 2017 22:55:12 +0200 Vlastimil Babka wrote: > On 06/02/2017 10:40 PM, Andrew Morton wrote: > > On Fri, 2 Jun 2017 22:31:47 +0200 Vlastimil Babka wrote: > >>> Perhaps we should be adding new prctl modes to select this new > >>> behaviour and leave the existing PR_SET_THP_DISABLE behaviour as-is? > >> > >> I think we can reasonably assume that most users of the prctl do just > >> the fork() & exec() thing, so they will be unaffected. > > > > That sounds optimistic. Perhaps people are using the current behaviour > > to set on particular mapping to MMF_DISABLE_THP, with > > > > prctl(PR_SET_THP_DISABLE) > > mmap() > > prctl(PR_CLR_THP_DISABLE) > > > > ? > > > > Seems a reasonable thing to do. > > Using madvise(MADV_NOHUGEPAGE) seems reasonabler to me, with the same > effect. And it's older (2.6.38). > > > But who knows - people do all sorts of > > inventive things. > > Yeah :( but we can hope they don't even know that the prctl currently > behaves they way it does - man page doesn't suggest it would, and most > of us in this thread found it surprising. Well. There might be such people and sometimes we do make people unhappy. it partly depends on how traumatic it would be to leave the current behaviour as-is. Have you evaluated such a patch? > >> And as usual, if > >> somebody does complain in the end, we revert and try the other way? > > > > But by then it's too late - the new behaviour will be out in the field. > > Revert in stable then? > But I don't think this patch should go to stable. I understand right > that CRIU will switch to the UFFDIO_COPY approach and doesn't need the > prctl change/new madvise anymore? What I mean is that the new behaviour will go out in 4.12 and it may be many months before we find out that we broke someone. By then, we can't go back because others may be assuming the new behaviour.