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Fri, 24 Apr 2020 07:57:56 +0000 (UTC) Received: from [10.36.113.138] (ovpn-113-138.ams2.redhat.com [10.36.113.138]) by smtp.corp.redhat.com (Postfix) with ESMTP id BB3A6100034E; Fri, 24 Apr 2020 07:57:50 +0000 (UTC) Subject: Re: [RFC PATCH 4/4] mm: Add PG_zero support To: Vlastimil Babka , Matthew Wilcox , Andrew Morton Cc: Alexander Duyck , Mel Gorman , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrea Arcangeli , Dan Williams , Dave Hansen , Michal Hocko , Alex Williamson References: <20200412090945.GA19582@open-light-1.localdomain> <20200412101223.GK21484@bombadil.infradead.org> <5eb37d79-6420-fcb9-2b4c-6cc6194afcd9@linux.intel.com> <20200413140537.eb674579cf8c71b4e20581ab@linux-foundation.org> <344a3a78-62ad-48fe-40cf-18993175d1e0@suse.cz> <20200423173700.b2c954b3960e4379a4f82e80@linux-foundation.org> <20200424004152.GD13910@bombadil.infradead.org> <09885eda-9772-4228-dece-3dfd42840580@suse.cz> From: David Hildenbrand Autocrypt: addr=david@redhat.com; prefer-encrypt=mutual; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: <09885eda-9772-4228-dece-3dfd42840580@suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 24.04.20 09:55, Vlastimil Babka wrote: > On 4/24/20 9:28 AM, David Hildenbrand wrote: >> On 24.04.20 02:41, Matthew Wilcox wrote: >>> On Thu, Apr 23, 2020 at 05:37:00PM -0700, Andrew Morton wrote: >>>> On Wed, 22 Apr 2020 16:09:00 +0200 Vlastimil Babka wrote: >>>>> Heh, I was quite sure that this is not the first time background zeroing is >>>>> proposed, so I went to google for it... and found that one BSD kernel actually >>>>> removed this functionality in 2016 [1] and this was one of the reasons. >>>>> >>>>> [1] >>>>> https://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/afd2da4dc9056ea79cdf15e8a9386a3d3998f33e >>>> >>>> Interesting. >>>> >>>> However this: >>>> >>>> - Pre-zeroing a page leads to a cold-cache case on-use, forcing the fault >>>> source (e.g. a userland program) to actually get the data from main >>>> memory in its likely immediate use of the faulted page, reducing >>>> performance. >>>> >>>> implies that BSD was zeroing with non-temporal stores which bypass the >>>> CPU cache. And which presumably invalidate any part of the target >>>> memory which was already in cache. We wouldn't do it that way so >>>> perhaps the results would differ. >>> >>> Or just that the page was zeroed far enough in advance that it fell out >>> of cache naturally. > > Agreed. > >>> I know Arjan looked at zeroing on free instead of zeroing on alloc, >>> and that didn't get merged (or even submitted afaik), so presumably the >>> results weren't good. >> >> We do have INIT_ON_FREE_DEFAULT_ON >> >> via >> >> commit 6471384af2a6530696fc0203bafe4de41a23c9ef >> Author: Alexander Potapenko >> Date: Thu Jul 11 20:59:19 2019 -0700 >> >> mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options >> >> which seems to do exactly that (although for a different use case) > > Yeah, except the security use case wants to do that immediately, while the > proposal here is a background thread. > Yes I know, this was just a comment regarding "Arjan looked at zeroing on free instead of zeroing on alloc". -- Thanks, David / dhildenb