From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: [PATCH] mm: make PR_SET_THP_DISABLE immediately active Date: Fri, 2 Jun 2017 12:50:59 -0700 Message-ID: <20170602125059.66209870607085b84c257593@linux-foundation.org> References: <1496415802-30944-1-git-send-email-rppt@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1496415802-30944-1-git-send-email-rppt-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org> Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Mike Rapoport Cc: Linux API , Michal Hocko , Vlastimil Babka , Andrea Arcangeli , Arnd Bergmann , "Kirill A. Shutemov" , Pavel Emelyanov , linux-mm , lkml , Michal Hocko List-Id: linux-api@vger.kernel.org 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?