From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.4 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C8A23C388F9 for ; Tue, 27 Oct 2020 11:05:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 70CFA21655 for ; Tue, 27 Oct 2020 11:05:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2898897AbgJ0LFX (ORCPT ); Tue, 27 Oct 2020 07:05:23 -0400 Received: from mx2.suse.de ([195.135.220.15]:43474 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2896940AbgJ0LFW (ORCPT ); Tue, 27 Oct 2020 07:05:22 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id A7CD1AAF1; Tue, 27 Oct 2020 11:05:21 +0000 (UTC) To: David Hildenbrand , Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Alexander Potapenko , Kees Cook , Michal Hocko , Mateusz Nosek References: <20201026173358.14704-1-vbabka@suse.cz> <20201026173358.14704-4-vbabka@suse.cz> <93ab79df-cf8c-294b-3ed1-8a563e4a452b@redhat.com> From: Vlastimil Babka Subject: Re: [PATCH 3/3] mm, page_alloc: reduce static keys in prep_new_page() Message-ID: <1fc7ec3a-367c-eb9f-1cb4-b9e015fea87c@suse.cz> Date: Tue, 27 Oct 2020 12:05:21 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.3 MIME-Version: 1.0 In-Reply-To: <93ab79df-cf8c-294b-3ed1-8a563e4a452b@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/27/20 10:10 AM, David Hildenbrand wrote: > On 26.10.20 18:33, Vlastimil Babka wrote: >> prep_new_page() will always zero a new page (regardless of __GFP_ZERO) when >> init_on_alloc is enabled, but will also always skip zeroing if the page was >> already zeroed on free by init_on_free or page poisoning. >> >> The latter check implemented by free_pages_prezeroed() can involve two >> different static keys. As prep_new_page() is really a hot path, let's introduce >> a single static key free_pages_not_prezeroed for this purpose and initialize it >> in init_mem_debugging(). > > Is this actually observable in practice? This smells like > micro-optimization to me. > > Also, I thought the whole reason for static keys is to have basically no > overhead at runtime, so I wonder if replacing two static key checks by a > single one actually makes *some* difference. You're right, the difference seems to be just a single NOP. The static key infrastructure seems to be working really well. (At least the asm inspection made me realize that kernel_poison_pages() is called unconditionally and the static key is checked inside, not inline so I'll be amending patch 2...) Initially I thought I would be reducing 3 keys to 1 in this patch, but I got the code wrong. So unless others think it's a readability improvements, we can drop this patch. Or we can also reconsider this whole optimization. If the point is to be paranoid and enable both init_on_free and init_on_alloc, should we trust that nobody wrote something after the clearing on free via use-after-free? :) Kees/Alex?