Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751189AbdFBUkl (ORCPT ); Fri, 2 Jun 2017 16:40:41 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:41380 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750918AbdFBUkk (ORCPT ); Fri, 2 Jun 2017 16:40:40 -0400 Date: Fri, 2 Jun 2017 13:40:38 -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: <20170602134038.13728cb77678ae1a7d7128a4@linux-foundation.org> In-Reply-To: <8a810c81-6a72-2af0-a450-6f03c71d8cca@suse.cz> References: <1496415802-30944-1-git-send-email-rppt@linux.vnet.ibm.com> <20170602125059.66209870607085b84c257593@linux-foundation.org> <8a810c81-6a72-2af0-a450-6f03c71d8cca@suse.cz> 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: 2645 Lines: 58 On Fri, 2 Jun 2017 22:31:47 +0200 Vlastimil Babka wrote: > On 06/02/2017 09:50 PM, Andrew Morton wrote: > > On Fri, 2 Jun 2017 18:03:22 +0300 "Mike Rapoport" wrote: > > > >> PR_SET_THP_DISABLE has a rather subtle semantic. It doesn't affect any > >> existing mapping because it only updated mm->def_flags which is a template > >> for new mappings. The mappings created after prctl(PR_SET_THP_DISABLE) have > >> VM_NOHUGEPAGE flag set. This can be quite surprising for all those > >> applications which do not do prctl(); fork() & exec() and want to control > >> their own THP behavior. > >> > >> Another usecase when the immediate semantic of the prctl might be useful is > >> a combination of pre- and post-copy migration of containers with CRIU. In > >> this case CRIU populates a part of a memory region with data that was saved > >> during the pre-copy stage. Afterwards, the region is registered with > >> userfaultfd and CRIU expects to get page faults for the parts of the region > >> that were not yet populated. However, khugepaged collapses the pages and > >> the expected page faults do not occur. > >> > >> In more general case, the prctl(PR_SET_THP_DISABLE) could be used as a > >> temporary mechanism for enabling/disabling THP process wide. > >> > >> Implementation wise, a new MMF_DISABLE_THP flag is added. This flag is > >> tested when decision whether to use huge pages is taken either during page > >> fault of at the time of THP collapse. > >> > >> It should be noted, that the new implementation makes PR_SET_THP_DISABLE > >> master override to any per-VMA setting, which was not the case previously. > >> > >> Fixes: a0715cc22601 ("mm, thp: add VM_INIT_DEF_MASK and PRCTL_THP_DISABLE") > > > > "Fixes" is a bit strong. I'd say "alters". And significantly altering > > the runtime behaviour of a three-year-old interface is rather a worry, > > no? > > > > 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. But who knows - people do all sorts of inventive things. > 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.