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[195.135.220.15]) by mx.google.com with ESMTPS id q6si15934236edg.394.2019.05.21.07.25.44 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 21 May 2019 07:25:44 -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 7E995AE91; Tue, 21 May 2019 14:25:43 +0000 (UTC) Date: Tue, 21 May 2019 16:25:41 +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: <20190521142541.GW32329@dhcp22.suse.cz> References: <20190514143537.10435-1-glider@google.com> <20190514143537.10435-4-glider@google.com> <20190517125916.GF1825@dhcp22.suse.cz> <20190517132542.GJ6836@dhcp22.suse.cz> <20190517140108.GK6836@dhcp22.suse.cz> <201905170925.6FD47DDFFF@keescook> <20190517171105.GT6836@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 Tue 21-05-19 16:18:37, Alexander Potapenko wrote: > On Fri, May 17, 2019 at 7:11 PM Michal Hocko wrote: > > > > On Fri 17-05-19 09:27:54, Kees Cook wrote: > > > On Fri, May 17, 2019 at 04:01:08PM +0200, Michal Hocko wrote: > > > > On Fri 17-05-19 15:37:14, Alexander Potapenko wrote: > > > > > > > > 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. > > > > > Ah, I see. We already do that, see the want_init_on_alloc() > > > > > implementation here: https://patchwork.kernel.org/patch/10943087/ > > > > > > > > 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? > > > > > When the kernel allocates an object from SLUB, and SLUB happens to be > > > > > short on free pages, it requests some from the page allocator. > > > > > Those pages are initialized by the page allocator > > > > > > > > ... when the feature is enabled ... > > > > > > > > > and split into objects. Finally SLUB initializes one of the available > > > > > objects and returns it back to the kernel. > > > > > Therefore the object is initialized twice for the first time (when it > > > > > comes directly from the page allocator). > > > > > This cost is however amortized by SLUB reusing the object after it's been freed. > > > > > > > > OK, I see what you mean now. Is there any way to special case the page > > > > allocation for this feature? E.g. your implementation tries to make this > > > > zeroying special but why cannot you simply do this > > > > > > > > > > > > struct page * > > > > ____alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid, > > > > nodemask_t *nodemask) > > > > { > > > > //current implementation > > > > } > > > > > > > > struct page * > > > > __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid, > > > > nodemask_t *nodemask) > > > > { > > > > if (your_feature_enabled) > > > > gfp_mask |= __GFP_ZERO; > > > > return ____alloc_pages_nodemask(gfp_mask, order, preferred_nid, > > > > nodemask); > > > > } > > > > > > > > and use ____alloc_pages_nodemask from the slab or other internal > > > > allocators? > Given that calling alloc_pages() with __GFP_NO_AUTOINIT doesn't > visibly improve the chosen benchmarks, > and the next patch in the series ("net: apply __GFP_NO_AUTOINIT to > AF_UNIX sk_buff allocations") only improves hackbench, > shall we maybe drop both patches altogether? Ohh, by all means. I was suggesting the same few emails ago. The above is just a hint on how to implement the feature on the page allocator level rather than hooking into the prep_new_page and add another branch to zero memory. -- Michal Hocko SUSE Labs