netdev - Re: [PATCH 1/1] mm: Fix struct page layout on 32-bit systems

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Date:   Fri, 16 Apr 2021 12:05:53 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Matteo Croce <mcroce@...ux.microsoft.com>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Arnd Bergmann <arnd@...nel.org>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 1/1] mm: Fix struct page layout on 32-bit systems

On Fri, Apr 16, 2021 at 07:32:35AM +0000, David Laight wrote:
> From: Matthew Wilcox <willy@...radead.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.