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

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Message-ID: <20210415182155.GD2531743@casper.infradead.org>
Date:   Thu, 15 Apr 2021 19:21:55 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     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 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.