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[195.135.220.15]) by mx.google.com with ESMTPS id l44si5017509edb.410.2019.05.17.06.25.43 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 17 May 2019 06:25:43 -0700 (PDT) Received-SPF: softfail (google.com: domain of transitioning mhocko@kernel.org does not designate 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=softfail (google.com: domain of transitioning mhocko@kernel.org does not designate 195.135.220.15 as permitted sender) smtp.mailfrom=mhocko@kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=kernel.org X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id D31A8AE5A; Fri, 17 May 2019 13:25:42 +0000 (UTC) Date: Fri, 17 May 2019 15:25:42 +0200 From: Michal Hocko To: Alexander Potapenko Cc: Kees Cook , Andrew Morton , Christoph Lameter , Kernel Hardening , Masahiro Yamada , James Morris , "Serge E. Hallyn" , Nick Desaulniers , Kostya Serebryany , Dmitry Vyukov , Sandeep Patil , Laura Abbott , Randy Dunlap , Jann Horn , Mark Rutland , Souptick Joarder , Matthew Wilcox , Linux Memory Management List , linux-security-module Subject: Re: [PATCH v2 3/4] gfp: mm: introduce __GFP_NO_AUTOINIT Message-ID: <20190517132542.GJ6836@dhcp22.suse.cz> References: <20190514143537.10435-1-glider@google.com> <20190514143537.10435-4-glider@google.com> <20190517125916.GF1825@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) 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 Fri 17-05-19 15:18:19, Alexander Potapenko wrote: > On Fri, May 17, 2019 at 2:59 PM Michal this flag Hocko > wrote: > > > > [It would be great to keep people involved in the previous version in the > > CC list] > Yes, I've been trying to keep everyone in the loop, but your email > fell through the cracks. > Sorry about that. No problem > > On Tue 14-05-19 16:35:36, Alexander Potapenko wrote: > > > When passed to an allocator (either pagealloc or SL[AOU]B), > > > __GFP_NO_AUTOINIT tells it to not initialize the requested memory if the > > > init_on_alloc boot option is enabled. This can be useful in the cases > > > newly allocated memory is going to be initialized by the caller right > > > away. > > > > > > __GFP_NO_AUTOINIT doesn't affect init_on_free behavior, except for SLOB, > > > where init_on_free implies init_on_alloc. > > > > > > __GFP_NO_AUTOINIT basically defeats the hardening against information > > > leaks provided by init_on_alloc, so one should use it with caution. > > > > > > This patch also adds __GFP_NO_AUTOINIT to alloc_pages() calls in SL[AOU]B. > > > Doing so is safe, because the heap allocators initialize the pages they > > > receive before passing memory to the callers. > > > > I still do not like the idea of a new gfp flag as explained in the > > previous email. People will simply use it incorectly or arbitrarily. > > We have that juicy experience from the past. > > Just to preserve some context, here's the previous email: > https://patchwork.kernel.org/patch/10907595/ > (plus the patch removing GFP_TEMPORARY for the curious ones: > https://lwn.net/Articles/729145/) Not only. GFP_REPEAT being another one and probably others I cannot remember from the top of my head. > > Freeing a memory is an opt-in feature and the slab allocator can already > > tell many (with constructor or GFP_ZERO) do not need it. > Sorry, I didn't understand this piece. Could you please elaborate? The allocator can assume that caches with a constructor will initialize the object so additional zeroying is not needed. GFP_ZERO should be self explanatory. > > So can we go without this gfp thing and see whether somebody actually > > finds a performance problem with the feature enabled and think about > > what can we do about it rather than add this maint. nightmare from the > > very beginning? > > There were two reasons to introduce this flag initially. > The first was double initialization of pages allocated for SLUB. Could you elaborate please? > However the benchmark results provided in this and the previous patch > don't show any noticeable difference - most certainly because the cost > of initializing the page is amortized. > The second one was to fine-tune hackbench, for which the slowdown > drops by a factor of 2. > But optimizing a mitigation for certain benchmarks is a questionable > measure, so maybe we could really go without it. Agreed. Over optimization based on an artificial workloads tend to be dubious IMHO. -- Michal Hocko SUSE Labs