From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f72.google.com (mail-wm0-f72.google.com [74.125.82.72]) by kanga.kvack.org (Postfix) with ESMTP id B9B5F6B0005 for ; Wed, 13 Jun 2018 03:51:28 -0400 (EDT) Received: by mail-wm0-f72.google.com with SMTP id l4-v6so1127651wmh.0 for ; Wed, 13 Jun 2018 00:51:28 -0700 (PDT) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id x42-v6si2304995edm.257.2018.06.13.00.51.26 for (version=TLS1 cipher=AES128-SHA bits=128/128); Wed, 13 Jun 2018 00:51:26 -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: Vlastimil Babka Message-ID: <5eb9a018-d5ac-5732-04f1-222c343b840a@suse.cz> Date: Wed, 13 Jun 2018 09:51:23 +0200 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 Cc: Jason Baron , 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 09:15 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? My point is that if somebody is relying on MADV_DONTNEED not affecting mlocked pages, the consequences will be unexpected data loss, not just extra page faults. >> 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. Right, but as it's explicitly described there, it makes it more likely that somebody is relying on it. >> 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. Right. Might be either this inconsistency, or a new flag.