Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754549AbbHXQWv (ORCPT ); Mon, 24 Aug 2015 12:22:51 -0400 Received: from mail-la0-f43.google.com ([209.85.215.43]:33780 "EHLO mail-la0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750941AbbHXQWt (ORCPT ); Mon, 24 Aug 2015 12:22:49 -0400 MIME-Version: 1.0 In-Reply-To: <20150824155503.GB17005@akamai.com> References: <20150820075611.GD4780@dhcp22.suse.cz> <20150820170309.GA11557@akamai.com> <20150821072552.GF23723@dhcp22.suse.cz> <20150821183132.GA12835@akamai.com> <55DB1C77.8070705@suse.cz> <55DB29EB.1000308@suse.cz> <20150824150912.GA17005@akamai.com> <20150824155503.GB17005@akamai.com> Date: Mon, 24 Aug 2015 19:22:47 +0300 Message-ID: Subject: Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT From: Konstantin Khlebnikov To: Eric B Munson Cc: Vlastimil Babka , Michal Hocko , Andrew Morton , Jonathan Corbet , "Kirill A. Shutemov" , Linux Kernel Mailing List , dri-devel , "linux-mm@kvack.org" , Linux API Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3872 Lines: 91 On Mon, Aug 24, 2015 at 6:55 PM, Eric B Munson wrote: > On Mon, 24 Aug 2015, Konstantin Khlebnikov wrote: > >> On Mon, Aug 24, 2015 at 6:09 PM, Eric B Munson wrote: >> > On Mon, 24 Aug 2015, Vlastimil Babka wrote: >> > >> >> On 08/24/2015 03:50 PM, Konstantin Khlebnikov wrote: >> >> >On Mon, Aug 24, 2015 at 4:30 PM, Vlastimil Babka wrote: >> >> >>On 08/24/2015 12:17 PM, Konstantin Khlebnikov wrote: >> >> >>>> >> >> >>>> >> >> >>>>I am in the middle of implementing lock on fault this way, but I cannot >> >> >>>>see how we will hanlde mremap of a lock on fault region. Say we have >> >> >>>>the following: >> >> >>>> >> >> >>>> addr = mmap(len, MAP_ANONYMOUS, ...); >> >> >>>> mlock(addr, len, MLOCK_ONFAULT); >> >> >>>> ... >> >> >>>> mremap(addr, len, 2 * len, ...) >> >> >>>> >> >> >>>>There is no way for mremap to know that the area being remapped was lock >> >> >>>>on fault so it will be locked and prefaulted by remap. How can we avoid >> >> >>>>this without tracking per vma if it was locked with lock or lock on >> >> >>>>fault? >> >> >>> >> >> >>> >> >> >>>remap can count filled ptes and prefault only completely populated areas. >> >> >> >> >> >> >> >> >>Does (and should) mremap really prefault non-present pages? Shouldn't it >> >> >>just prepare the page tables and that's it? >> >> > >> >> >As I see mremap prefaults pages when it extends mlocked area. >> >> > >> >> >Also quote from manpage >> >> >: If the memory segment specified by old_address and old_size is locked >> >> >: (using mlock(2) or similar), then this lock is maintained when the segment is >> >> >: resized and/or relocated. As a consequence, the amount of memory locked >> >> >: by the process may change. >> >> >> >> Oh, right... Well that looks like a convincing argument for having a >> >> sticky VM_LOCKONFAULT after all. Having mremap guess by scanning >> >> existing pte's would slow it down, and be unreliable (was the area >> >> completely populated because MLOCK_ONFAULT was not used or because >> >> the process aulted it already? Was it not populated because >> >> MLOCK_ONFAULT was used, or because mmap(MAP_LOCKED) failed to >> >> populate it all?). >> > >> > Given this, I am going to stop working in v8 and leave the vma flag in >> > place. >> > >> >> >> >> The only sane alternative is to populate always for mremap() of >> >> VM_LOCKED areas, and document this loss of MLOCK_ONFAULT information >> >> as a limitation of mlock2(MLOCK_ONFAULT). Which might or might not >> >> be enough for Eric's usecase, but it's somewhat ugly. >> >> >> > >> > I don't think that this is the right solution, I would be really >> > surprised as a user if an area I locked with MLOCK_ONFAULT was then >> > fully locked and prepopulated after mremap(). >> >> If mremap is the only problem then we can add opposite flag for it: >> >> "MREMAP_NOPOPULATE" >> - do not populate new segment of locked areas >> - do not copy normal areas if possible (anonymous/special must be copied) >> >> addr = mmap(len, MAP_ANONYMOUS, ...); >> mlock(addr, len, MLOCK_ONFAULT); >> ... >> addr2 = mremap(addr, len, 2 * len, MREMAP_NOPOPULATE); >> ... >> > > But with this, the user must remember what areas are locked with > MLOCK_LOCKONFAULT and which are locked the with prepopulate so the > correct mremap flags can be used. > Yep. Shouldn't be hard. You anyway have to do some changes in user-space. Much simpler for users-pace solution is a mm-wide flag which turns all further mlocks and MAP_LOCKED into lock-on-fault. Something like mlockall(MCL_NOPOPULATE_LOCKED). -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/