Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751305AbdFBUct (ORCPT ); Fri, 2 Jun 2017 16:32:49 -0400 Received: from mx2.suse.de ([195.135.220.15]:48571 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750971AbdFBUcs (ORCPT ); Fri, 2 Jun 2017 16:32:48 -0400 Subject: Re: [PATCH] mm: make PR_SET_THP_DISABLE immediately active To: Andrew Morton , Mike Rapoport References: <1496415802-30944-1-git-send-email-rppt@linux.vnet.ibm.com> <20170602125059.66209870607085b84c257593@linux-foundation.org> Cc: Linux API , Michal Hocko , Andrea Arcangeli , Arnd Bergmann , "Kirill A. Shutemov" , Pavel Emelyanov , linux-mm , lkml , Michal Hocko From: Vlastimil Babka Message-ID: <8a810c81-6a72-2af0-a450-6f03c71d8cca@suse.cz> Date: Fri, 2 Jun 2017 22:31:47 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 MIME-Version: 1.0 In-Reply-To: <20170602125059.66209870607085b84c257593@linux-foundation.org> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2130 Lines: 40 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. And as usual, if somebody does complain in the end, we revert and try the other way?