Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 16 Apr 2021 12:05:53 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     David Laight <David.Laight@aculab.com>
Cc:     Jesper Dangaard Brouer <brouer@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>,
        Ilias Apalodimas <ilias.apalodimas@linaro.org>,
        Matteo Croce <mcroce@linux.microsoft.com>,
        Grygorii Strashko <grygorii.strashko@ti.com>,
        Arnd Bergmann <arnd@kernel.org>, Christoph Hellwig <hch@lst.de>
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>
Precedence: bulk

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.