Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751242AbdFBTvD (ORCPT ); Fri, 2 Jun 2017 15:51:03 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:38538 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750918AbdFBTvB (ORCPT ); Fri, 2 Jun 2017 15:51:01 -0400 Date: Fri, 2 Jun 2017 12:50:59 -0700 From: Andrew Morton To: "Mike Rapoport" Cc: Linux API , Michal Hocko , Vlastimil Babka , 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: <20170602125059.66209870607085b84c257593@linux-foundation.org> In-Reply-To: <1496415802-30944-1-git-send-email-rppt@linux.vnet.ibm.com> References: <1496415802-30944-1-git-send-email-rppt@linux.vnet.ibm.com> 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: 1834 Lines: 35 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?