From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f71.google.com (mail-pl0-f71.google.com [209.85.160.71]) by kanga.kvack.org (Postfix) with ESMTP id ED2DC6B0006 for ; Wed, 13 Jun 2018 02:32:26 -0400 (EDT) Received: by mail-pl0-f71.google.com with SMTP id 89-v6so955870plc.1 for ; Tue, 12 Jun 2018 23:32:26 -0700 (PDT) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id n6-v6si2130552pla.12.2018.06.12.23.32.24 for (version=TLS1 cipher=AES128-SHA bits=128/128); Tue, 12 Jun 2018 23:32:25 -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> From: Vlastimil Babka Message-ID: <0daccb7c-f642-c5ce-ca7a-3b3e69025a1e@suse.cz> Date: Wed, 13 Jun 2018 08:32:19 +0200 MIME-Version: 1.0 In-Reply-To: <5a9398f4-453c-5cb5-6bbc-f20c3affc96a@akamai.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Jason Baron , Michal Hocko 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 06/12/2018 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. The madvise manpage is also quite specific about not allowing MADV_DONTNEED and MADV_FREE for locked pages. 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? >> 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? > Thanks, > > -Jason > -- > To unsubscribe from this list: send the line "unsubscribe linux-api" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html >