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

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Message-ID: <20210419063441.GA18787@lst.de>
Date:   Mon, 19 Apr 2021 08:34:41 +0200
From:   Christoph Hellwig <hch@....de>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        David Laight <David.Laight@...lab.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 04:27:55PM +0100, Matthew Wilcox wrote:
> On Thu, Apr 15, 2021 at 08:08:32PM +0200, Jesper Dangaard Brouer wrote:
> > See below patch.  Where I swap32 the dma address to satisfy
> > page->compound having bit zero cleared. (It is the simplest fix I could
> > come up with).
> 
> I think this is slightly simpler, and as a bonus code that assumes the
> old layout won't compile.

So, why do we even do this crappy overlay of a dma address?  This just
all seems like a giant hack.  Random subsystems should not just steal
a few struct page fields as that just turns into the desasters like the
one we've seen here or probably something worse next time.