From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f199.google.com (mail-wr0-f199.google.com [209.85.128.199]) by kanga.kvack.org (Postfix) with ESMTP id E908A6B0005 for ; Wed, 13 Jun 2018 03:15:58 -0400 (EDT) Received: by mail-wr0-f199.google.com with SMTP id j14-v6so1036996wro.7 for ; Wed, 13 Jun 2018 00:15:58 -0700 (PDT) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id e36-v6si1899585eda.181.2018.06.13.00.15.57 for (version=TLS1 cipher=AES128-SHA bits=128/128); Wed, 13 Jun 2018 00:15:57 -0700 (PDT) Date: Wed, 13 Jun 2018 09:15:52 +0200 From: Michal Hocko Subject: Re: [PATCH] mm/madvise: allow MADV_DONTNEED to free memory that is MLOCK_ONFAULT Message-ID: <20180613071552.GD13364@dhcp22.suse.cz> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0daccb7c-f642-c5ce-ca7a-3b3e69025a1e@suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka 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 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. -- Michal Hocko SUSE Labs