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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id F07CBCA0EC6 for ; Mon, 11 Sep 2023 22:22:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1376521AbjIKWTw (ORCPT ); Mon, 11 Sep 2023 18:19:52 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39412 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237630AbjIKNDT (ORCPT ); Mon, 11 Sep 2023 09:03:19 -0400 Received: from smtp-out2.suse.de (smtp-out2.suse.de [IPv6:2001:67c:2178:6::1d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 57420DF for ; Mon, 11 Sep 2023 06:03:14 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out2.suse.de (Postfix) with ESMTPS id 11AB11F891; Mon, 11 Sep 2023 13:03:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1694437393; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=+lkCsOrmvgMZKwdhoCy1NavBv3f9A5WVMY+v90BVt4s=; b=DiGitxDw/pQ+mU4nN1HJ2KIq6mOYkeHtb2wMCjQm3rgm+YXJGw7ySwLfsfMTpuI9fH7h2a LX4b6JRDxzY4hKG1Ev/xLnveCPt5//NYkvcZONnMD7AlQzj0zN2CXW26vs5ynKEqxij24+ QwIraQZ/hydiUVZi3Yc9ShRs3vM01O8= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id E67F813780; Mon, 11 Sep 2023 13:03:12 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id V/VJNRAQ/2RnHwAAMHmgww (envelope-from ); Mon, 11 Sep 2023 13:03:12 +0000 Date: Mon, 11 Sep 2023 15:03:12 +0200 From: Michal Hocko To: Matthew Wilcox Cc: "zhaoyang.huang" , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Zhaoyang Huang , ke.wang@unisoc.com Subject: Re: [PATCH] mm: remove redundant clear page when CONFIG_INIT_ON_ALLOC_DEFAULT_ON configured Message-ID: References: <20230911104906.2058503-1-zhaoyang.huang@unisoc.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 11-09-23 13:47:03, Matthew Wilcox wrote: > On Mon, Sep 11, 2023 at 02:12:25PM +0200, Michal Hocko wrote: > > On Mon 11-09-23 18:49:06, zhaoyang.huang wrote: > > > From: Zhaoyang Huang > > > > > > There will be redundant clear page within vma_alloc_zeroed_movable_folio > > > when CONFIG_INIT_ON_ALLOC_DEFAULT_ON is on. Remove it by judging related > > > configs. > > > > Thanks for spotting this. I suspect this is a fix based on a code review > > rather than a real performance issue, right? It is always good to > > mention that. From a very quick look it seems that many architectures > > just definte vma_alloc_zeroed_movable_folio to use __GFP_ZERO so they > > are not affected by this. This means that only a subset of architectures > > are really affected. This is an important information as well. > > Finally I think it would be more appropriate to mention that the double > > initialization is done when init_on_alloc is enabled rather than > > referring to the above config option which only controls whether the > > functionality is enabled by default. > > This may well be an unsaafe change to make. We're not just zeroing the > page, we're calling clear_user_highpage() which tells the architecture > which virtual address the page will be mapped at. It could be that > skipping the zeroing ("because the page is already zero") isn't enough; > there will be traces of the former contents of some page in the D-cache > for this address. I haven't realized this difference between clear_user_highpage and kernel_init_pages which is used by the page allocator. Thanks for pointing this out! > > Or it might just be an optimisation. The description of clear_user_page() > isn't entirely clear; the port may be relying on clear_user_page() > to have flushed the dcache aliases. > > At this point, I don't think this patch is worth the risk. Agreed! Based on that I take my ack back. > My mind is > changable on this, but I think we'd need buy-in from ARM, SH and Xtensa > (who directly define clear_user_highpage()) as well as Arc, csky, ia64, > m68k, mips, nios2, parisc, powerpc, sparc who all seem to have non-trivial > clear_user_page() implementations. -- Michal Hocko SUSE Labs