From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f70.google.com (mail-wm0-f70.google.com [74.125.82.70]) by kanga.kvack.org (Postfix) with ESMTP id 7B9206B0003 for ; Fri, 15 Jun 2018 15:36:14 -0400 (EDT) Received: by mail-wm0-f70.google.com with SMTP id w21-v6so1757357wmc.4 for ; Fri, 15 Jun 2018 12:36:14 -0700 (PDT) Received: from mx0b-00190b01.pphosted.com (mx0b-00190b01.pphosted.com. [2620:100:9005:57f::1]) by mx.google.com with ESMTPS id x20-v6si7317651wrd.284.2018.06.15.12.36.12 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 15 Jun 2018 12:36:13 -0700 (PDT) Subject: Re: [PATCH] mm/madvise: allow MADV_DONTNEED to free memory that is MLOCK_ONFAULT References: <1528484212-7199-1-git-send-email-jbaron@akamai.com> <20180611072005.GC13364@dhcp22.suse.cz> <4c4de46d-c55a-99a8-469f-e1e634fb8525@akamai.com> <20180611150330.GQ13364@dhcp22.suse.cz> <775adf2d-140c-1460-857f-2de7b24bafe7@akamai.com> <20180612074646.GS13364@dhcp22.suse.cz> <5a9398f4-453c-5cb5-6bbc-f20c3affc96a@akamai.com> <0daccb7c-f642-c5ce-ca7a-3b3e69025a1e@suse.cz> <20180613071552.GD13364@dhcp22.suse.cz> From: Jason Baron Message-ID: <7a671035-92dc-f9c0-aa7b-ff916d556e82@akamai.com> Date: Fri, 15 Jun 2018 15:36:07 -0400 MIME-Version: 1.0 In-Reply-To: <20180613071552.GD13364@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko , Vlastimil Babka Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Joonsoo Kim , Mel Gorman , "Kirill A. Shutemov" , linux-api@vger.kernel.org, emunson@mgebm.net On 06/13/2018 03:15 AM, Michal Hocko wrote: > On Wed 13-06-18 08:32:19, Vlastimil Babka wrote: >> On 06/12/2018 04:11 PM, Jason Baron wrote: >>> >>> >>> On 06/12/2018 03:46 AM, Michal Hocko wrote: >>>> On Mon 11-06-18 12:23:58, Jason Baron wrote: >>>>> On 06/11/2018 11:03 AM, Michal Hocko wrote: >>>>>> So can we start discussing whether we want to allow MADV_DONTNEED on >>>>>> mlocked areas and what downsides it might have? Sure it would turn the >>>>>> strong mlock guarantee to have the whole vma resident but is this >>>>>> acceptable for something that is an explicit request from the owner of >>>>>> the memory? >>>>>> >>>>> >>>>> If its being explicity requested by the owner it makes sense to me. I >>>>> guess there could be a concern about this breaking some userspace that >>>>> relied on MADV_DONTNEED not freeing locked memory? >>>> >>>> Yes, this is always the fear when changing user visible behavior. I can >>>> imagine that a userspace allocator calling MADV_DONTNEED on free could >>>> break. The same would apply to MLOCK_ONFAULT/MCL_ONFAULT though. We >>>> have the new flag much shorter so the probability is smaller but the >>>> problem is very same. So I _think_ we should treat both the same because >>>> semantically they are indistinguishable from the MADV_DONTNEED POV. Both >>>> remove faulted and mlocked pages. Mlock, once applied, should guarantee >>>> no later major fault and MADV_DONTNEED breaks that obviously. >> >> I think more concerning than guaranteeing no later major fault is >> possible data loss, e.g. replacing data with zero-filled pages. > > But MADV_DONTNEED is an explicit call for data loss. Or do I miss your > point? > >> The madvise manpage is also quite specific about not allowing >> MADV_DONTNEED and MADV_FREE for locked pages. > > Yeah, but that seems to describe the state of the art rather than > explain why. > >> So I don't think we should risk changing that for all mlocked pages. >> Maybe we can risk MCL_ONFAULT, since it's relatively new and has few users? > > That is what Jason wanted but I argued that the two are the same from > MADV_DONTNEED point of view. I do not see how treating them differently > would be less confusing or error prone. It's new so we can make it > behave differently is certainly not an argument. > >>>> So the more I think about it the more I am worried about this but I am >>>> more and more convinced that making ONFAULT special is just a wrong way >>>> around this. >>>> >>> >>> Ok, I share the concern that there is a chance that userspace is relying >>> on MADV_DONTNEED not free'ing locked memory. In that case, what if we >>> introduce a MADV_DONTNEED_FORCE, which does everything that >>> MADV_DONTNEED currently does but in addition will also free mlock areas. >>> That way there is no concern about breaking something. >> >> A new niche case flag? Sad :( >> >> BTW I didn't get why we should allow this for MADV_DONTNEED but not >> MADV_FREE. Can you expand on that? > > Well, I wanted to bring this up as well. I guess this would require some > more hacks to handle the reclaim path correctly because we do rely on > VM_LOCK at many places for the lazy mlock pages culling. > The point of not allowing MADV_FREE on mlock'd pages for me was that with mlock and even MLOCK_ON_FAULT, one can always can always determine if a page is present or not (and thus avoid the major fault). Allowing MADV_FREE on lock'd pages breaks that assumption. Thanks, -Jason