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

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Date:   Thu, 15 Apr 2021 21:11:56 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Matthew Wilcox' <willy@...radead.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>
CC:     "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

From: Matthew Wilcox <willy@...radead.org>
> Sent: 15 April 2021 19:22
> 
> On Thu, Apr 15, 2021 at 08:08:32PM +0200, Jesper Dangaard Brouer wrote:
> > +static inline
> > +dma_addr_t page_pool_dma_addr_read(dma_addr_t dma_addr)
> > +{
> > +	/* Workaround for storing 64-bit DMA-addr on 32-bit machines in struct
> > +	 * page.  The page->dma_addr share area with page->compound_head which
> > +	 * use bit zero to mark compound pages. This is okay, as DMA-addr are
> > +	 * aligned pointers which have bit zero cleared.
> > +	 *
> > +	 * In the 32-bit case, page->compound_head is 32-bit.  Thus, when
> > +	 * dma_addr_t is 64-bit it will be located in top 32-bit.  Solve by
> > +	 * swapping dma_addr 32-bit segments.
> > +	 */
> > +#ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
> 
> #if defined(CONFIG_ARCH_DMA_ADDR_T_64BIT) && defined(__BIG_ENDIAN)
> otherwise you'll create the problem on ARM that you're avoiding on PPC ...
> 
> I think you want to delete the word '_read' from this function name because
> you're using it for both read and write.

I think I'd use explicit dma_addr_hi and dma_addr_lo and
separate read/write functions just to make absolutely sure
nothing picks up the swapped value.

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.

	David

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