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 5C26BCA0EC4 for ; Mon, 11 Sep 2023 21:40:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1350410AbjIKViI (ORCPT ); Mon, 11 Sep 2023 17:38:08 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57582 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237394AbjIKMrY (ORCPT ); Mon, 11 Sep 2023 08:47:24 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 955CCE40 for ; Mon, 11 Sep 2023 05:47:19 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=AT/HwmSFNvPgul/c1kKp5PbdGjEUzpII4wStP5rXfVI=; b=GAGPBrS4O64uc1P1Dkt0dT5nbd wkftLVuzMLe1jQYqNe1N5/9Cs96OcwNv/MClUqUZ91ZRefOLLHW2HbHfd0oQKpWwbTDl1vC3ZJ3PY f1shLDG53/7qYiki3xfy5Yg1KsPVFOZWrTklRGEjq7FV01u/3rWvmfYipKn22+fiXUUPkVQ3P0/UD ywAyX66FLyzCCCs2349Z4YQdi+T1EojvXQkxelwPYxxCYretVYjxO3Ai5Gz+SzMssFYkheGfW0I7l xv8+AFC++Pj0xJIGNdWwpKvuXaA7oQUD7LUaFppzv91Oqxf1qhfILlJUBIqG9Lo7Zt4q4ecnrebZH ywR7pbpg==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1qfgJj-0010FI-FL; Mon, 11 Sep 2023 12:47:03 +0000 Date: Mon, 11 Sep 2023 13:47:03 +0100 From: Matthew Wilcox To: Michal Hocko 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, 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. 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. 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.