From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <SRS0=vApQ=JN=lists.ozlabs.org=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@kernel.org>
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=-3.5 required=3.0 tests=BAYES_00,DKIM_INVALID,
	DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,
	SPF_PASS,URIBL_BLOCKED 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 7348EC433B4
	for <linuxppc-dev@archiver.kernel.org>; Fri, 16 Apr 2021 11:06:50 +0000 (UTC)
Received: from lists.ozlabs.org (lists.ozlabs.org [112.213.38.117])
	(using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
	(No client certificate requested)
	by mail.kernel.org (Postfix) with ESMTPS id D9AEA6113B
	for <linuxppc-dev@archiver.kernel.org>; Fri, 16 Apr 2021 11:06:49 +0000 (UTC)
DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D9AEA6113B
Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org
Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org
Received: from boromir.ozlabs.org (localhost [IPv6:::1])
	by lists.ozlabs.org (Postfix) with ESMTP id 4FMD1N2sTSz3bys
	for <linuxppc-dev@archiver.kernel.org>; Fri, 16 Apr 2021 21:06:48 +1000 (AEST)
Authentication-Results: lists.ozlabs.org;
	dkim=fail reason="signature verification failed" (2048-bit key; secure) header.d=infradead.org header.i=@infradead.org header.a=rsa-sha256 header.s=casper.20170209 header.b=hh9zQfxD;
	dkim-atps=neutral
Authentication-Results: lists.ozlabs.org;
 spf=none (no SPF record) smtp.mailfrom=infradead.org
 (client-ip=2001:8b0:10b:1236::1; helo=casper.infradead.org;
 envelope-from=willy@infradead.org; receiver=<UNKNOWN>)
Authentication-Results: lists.ozlabs.org; dkim=pass (2048-bit key;
 secure) header.d=infradead.org header.i=@infradead.org header.a=rsa-sha256
 header.s=casper.20170209 header.b=hh9zQfxD; 
 dkim-atps=neutral
Received: from casper.infradead.org (casper.infradead.org
 [IPv6:2001:8b0:10b:1236::1])
 (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)
 key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256)
 (No client certificate requested)
 by lists.ozlabs.org (Postfix) with ESMTPS id 4FMD0t6r3Yz30Hk
 for <linuxppc-dev@lists.ozlabs.org>; Fri, 16 Apr 2021 21:06:22 +1000 (AEST)
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=q8yzRZdY2wah1G/yCTkxWNbB0jFfZUvqQ3uEjV6WRbg=; b=hh9zQfxDvG2X4xo9Vk3zOGv+gZ
 QKntKgbYQxOmJbb/qwMrADTgbL8yz3oUhd3HEVZJgrY81Z+RWvtUKKoC8013lyugG2k8O7umfbJam
 gAhDbhGAPve6oN0MKZsJdwlfW5fC1MWvd6G5LMajXM5TqmIFfFtrkgHcaAZHgvXMnGtFGhwT/4HmK
 CsjhiBL3nsVtLo7E7Dvy9fj8xomDDJge0rOCzwzXKycyeIlDt/8ml1XDq6Qx+KbmBhQ5d1+BhQ6W5
 Piq3UrI2NLCuHxk3SVM4ryUk+gw6UwwBM7IiZJ4NrS7iOhjfU6YhJB+tUEtO+rntTBicYBF62ikeR
 Oi9OJ1dA==;
Received: from willy by casper.infradead.org with local (Exim 4.94 #2 (Red Hat
 Linux)) id 1lXMIL-009rDn-92; Fri, 16 Apr 2021 11:06:01 +0000
Date: Fri, 16 Apr 2021 12:05:53 +0100
From: Matthew Wilcox <willy@infradead.org>
To: David Laight <David.Laight@aculab.com>
Subject: Re: [PATCH 1/1] mm: Fix struct page layout on 32-bit systems
Message-ID: <20210416110553.GH2531743@casper.infradead.org>
References: <20210414101044.19da09df@carbon>
 <20210414115052.GS2531743@casper.infradead.org>
 <20210414211322.3799afd4@carbon>
 <20210414213556.GY2531743@casper.infradead.org>
 <a50c3156fe8943ef964db4345344862f@AcuMS.aculab.com>
 <20210415200832.32796445@carbon>
 <20210415182155.GD2531743@casper.infradead.org>
 <5179a01a462f43d6951a65de2a299070@AcuMS.aculab.com>
 <20210415222211.GG2531743@casper.infradead.org>
 <f51e1aa98cb94880a236d58c75c20994@AcuMS.aculab.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <f51e1aa98cb94880a236d58c75c20994@AcuMS.aculab.com>
X-BeenThere: linuxppc-dev@lists.ozlabs.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Linux on PowerPC Developers Mail List <linuxppc-dev.lists.ozlabs.org>
List-Unsubscribe: <https://lists.ozlabs.org/options/linuxppc-dev>,
 <mailto:linuxppc-dev-request@lists.ozlabs.org?subject=unsubscribe>
List-Archive: <http://lists.ozlabs.org/pipermail/linuxppc-dev/>
List-Post: <mailto:linuxppc-dev@lists.ozlabs.org>
List-Help: <mailto:linuxppc-dev-request@lists.ozlabs.org?subject=help>
List-Subscribe: <https://lists.ozlabs.org/listinfo/linuxppc-dev>,
 <mailto:linuxppc-dev-request@lists.ozlabs.org?subject=subscribe>
Cc: Arnd Bergmann <arnd@kernel.org>,
 Grygorii Strashko <grygorii.strashko@ti.com>,
 "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
 Ilias Apalodimas <ilias.apalodimas@linaro.org>,
 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
 "linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>,
 "linux-mm@kvack.org" <linux-mm@kvack.org>,
 Jesper Dangaard Brouer <brouer@redhat.com>,
 Matteo Croce <mcroce@linux.microsoft.com>,
 "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
 Christoph Hellwig <hch@lst.de>,
 "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>
Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org
Sender: "Linuxppc-dev"
 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

On Fri, Apr 16, 2021 at 07:32:35AM +0000, David Laight wrote:
> From: Matthew Wilcox <willy@infradead.org>
> > Sent: 15 April 2021 23:22
> > 
> > On Thu, Apr 15, 2021 at 09:11:56PM +0000, David Laight wrote:
> > > Isn't it possible to move the field down one long?
> > > This might require an explicit zero - but this is not a common
> > > code path - the extra write will be noise.
> > 
> > Then it overlaps page->mapping.  See emails passim.
> 
> The rules on overlaps make be wonder if every 'long'
> should be in its own union.

That was what we used to have.  It was worse.

> The comments would need to say when each field is used.
> It would, at least, make these errors less common.
> 
> That doesn't solve the 64bit dma_addr though.
> 
> Actually rather that word-swapping dma_addr on 32bit BE
> could you swap over the two fields it overlays with.
> That might look messy in the .h, but it doesn't require
> an accessor function to do the swap - easily missed.

No.