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 B4CBCC433EF for ; Thu, 12 May 2022 00:22:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1349628AbiELAWN (ORCPT ); Wed, 11 May 2022 20:22:13 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38928 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242707AbiELAWK (ORCPT ); Wed, 11 May 2022 20:22:10 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1A79C165D65 for ; Wed, 11 May 2022 17:22:09 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id A995E61E25 for ; Thu, 12 May 2022 00:22:08 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0A071C340EE; Thu, 12 May 2022 00:22:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1652314928; bh=seXO4v2zX3vcG/CFv9RKV/1XIKUC+Q16x5rmcL8DhCM=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=kN/EJ4nonEfmbs/Ic/G3qvfOZXm70wH1xUaFAcbUau0GlW+56XCzk2BfyNvCxXE2m 3LKjaID3cWwtNMOio4awk7IpO/IQuxYSTsF7fLQaQYx3QWHAe7VgJnz44ep6buRWsy gf0oYg76MJIXTPuZzfpHpQ/e5nPXrs9Q29jwDZlPEFi1CoToMqgnMzvm6/+Dmr7XAy mC0ihbSHiStpKC+jjzIMEBltwbo4O9Q+tDepFjBCQsSB/1u4YVvYPhWvfbobJ6CCgQ pTnlKvoTqpbMDB4VlmJWobr+4OflLxvHgXMsx8ORdUGn5OVHKOlt2tbaeHzR7JXp00 dC2gbu0B6NQHA== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 7FB875C05FC; Wed, 11 May 2022 17:22:07 -0700 (PDT) Date: Wed, 11 May 2022 17:22:07 -0700 From: "Paul E. McKenney" To: John Hubbard Cc: Minchan Kim , Andrew Morton , linux-mm , LKML , John Dias , David Hildenbrand Subject: Re: [PATCH v4] mm: fix is_pinnable_page against on cma page Message-ID: <20220512002207.GJ1790663@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <2ffa7670-04ea-bb28-28f8-93a9b9eea7e8@nvidia.com> <54b5d177-f2f4-cef2-3a68-cd3b0b276f86@nvidia.com> <8f083802-7ab0-15ec-b37d-bc9471eea0b1@nvidia.com> <20220511234534.GG1790663@paulmck-ThinkPad-P17-Gen-1> <0d90390c-3624-4f93-f8bd-fb29e92237d3@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <0d90390c-3624-4f93-f8bd-fb29e92237d3@nvidia.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 11, 2022 at 05:12:32PM -0700, John Hubbard wrote: > On 5/11/22 16:57, John Hubbard wrote: > > On 5/11/22 16:45, Paul E. McKenney wrote: > > > > > > > > Well no, because the "&" operation is a single operation on the CPU, and > > > > isn't going to get split up like that. > > > > > > Chiming in a bit late... > > > > Much appreciated! > > > > > > > > The usual way that this sort of thing causes trouble is if there is a > > > single store instruction that changes the value from MIGRATE_ISOLATE > > > to MIGRATE_CMA, and if the compiler decides to fetch twice, AND twice, > > > > Doing an AND twice for "x & constant" this definitely blows my mind. Is > > nothing sacred? :) > > > > > and then combine the results.  This could give a zero outcome where the > > > underlying variable never had the value zero. > > > > > > Is this sort of thing low probability? > > > > > > Definitely. > > > > > > Isn't this sort of thing prohibited? > > > > > > Definitely not. > > > > > > So what you have will likely work for at least a while longer, but it > > > is not guaranteed and it forces you to think a lot harder about what > > > the current implementations of the compiler can and cannot do to you. > > > > > > The following LWN article goes through some of the possible optimizations > > > (vandalisms?) in this area: https://lwn.net/Articles/793253/ > > > > > > > hmm, I don't think we hit any of those  cases, do we? Because here, the > > "write" side is via a non-inline function that I just don't believe the > > compiler is allowed to call twice. Or is it? > > > > Minchan's earlier summary: > > > > CPU 0                         CPU1 > > > > > >                               set_pageblock_migratetype(MIGRATE_ISOLATE) > > > > if (get_pageblock_migrate(page) & MIGRATE_CMA) > > > >                               set_pageblock_migratetype(MIGRATE_CMA) > > > > if (get_pageblock_migrate(page) & MIGRATE_ISOLATE) > > > > ...where set_pageblock_migratetype() is not inline. > > > > thanks, > > Let me try to say this more clearly: I don't think that the following > __READ_ONCE() statement can actually help anything, given that > get_pageblock_migratetype() is non-inlined: > > + int __mt = get_pageblock_migratetype(page); > + int mt = __READ_ONCE(__mt); > + > + if (mt & (MIGRATE_CMA | MIGRATE_ISOLATE)) > + return false; > > > Am I missing anything here? In the absence of future aggression from link-time optimizations (LTO), you are missing nothing. Thanx, Paul