From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f69.google.com (mail-wm0-f69.google.com [74.125.82.69]) by kanga.kvack.org (Postfix) with ESMTP id B1E756B0275 for ; Tue, 12 Jun 2018 03:46:50 -0400 (EDT) Received: by mail-wm0-f69.google.com with SMTP id g73-v6so6006786wmc.5 for ; Tue, 12 Jun 2018 00:46:50 -0700 (PDT) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id t12-v6si489241edk.129.2018.06.12.00.46.49 for (version=TLS1 cipher=AES128-SHA bits=128/128); Tue, 12 Jun 2018 00:46:49 -0700 (PDT) Date: Tue, 12 Jun 2018 09:46:46 +0200 From: Michal Hocko Subject: Re: [PATCH] mm/madvise: allow MADV_DONTNEED to free memory that is MLOCK_ONFAULT Message-ID: <20180612074646.GS13364@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <775adf2d-140c-1460-857f-2de7b24bafe7@akamai.com> Sender: owner-linux-mm@kvack.org List-ID: To: Jason Baron Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Vlastimil Babka , Joonsoo Kim , Mel Gorman , "Kirill A. Shutemov" , linux-api@vger.kernel.org, emunson@mgebm.net 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. 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. -- Michal Hocko SUSE Labs