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 899AA6B0003 for ; Wed, 20 Jun 2018 07:00:31 -0400 (EDT) Received: by mail-wr0-f199.google.com with SMTP id f7-v6so2241436wrq.19 for ; Wed, 20 Jun 2018 04:00:31 -0700 (PDT) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id m18-v6si1006303edq.440.2018.06.20.04.00.27 for (version=TLS1 cipher=AES128-SHA bits=128/128); Wed, 20 Jun 2018 04:00:27 -0700 (PDT) Date: Wed, 20 Jun 2018 13:00:22 +0200 From: Michal Hocko Subject: Re: [PATCH] mm/madvise: allow MADV_DONTNEED to free memory that is MLOCK_ONFAULT Message-ID: <20180620110022.GK13685@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> <20180613071552.GD13364@dhcp22.suse.cz> <7a671035-92dc-f9c0-aa7b-ff916d556e82@akamai.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7a671035-92dc-f9c0-aa7b-ff916d556e82@akamai.com> Sender: owner-linux-mm@kvack.org List-ID: To: Jason Baron Cc: Vlastimil Babka , 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 Fri 15-06-18 15:36:07, Jason Baron wrote: > > > On 06/13/2018 03:15 AM, Michal Hocko wrote: > > On Wed 13-06-18 08:32:19, Vlastimil Babka wrote: [...] > >> 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. But once you have called MADV_FREE you cannot assume anything about the content until you touch the memory again. So you can safely assume a major fault for the worst case. Btw. why knowing whether you major fault is important in the first place? What is an application going to do about that information? -- Michal Hocko SUSE Labs