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

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Date:   Sat, 17 Apr 2021 12:31:37 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Grygorii Strashko <grygorii.strashko@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        David Laight <David.Laight@...lab.com>,
        Matteo Croce <mcroce@...ux.microsoft.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        Christoph Hellwig <hch@....de>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/1] mm: Fix struct page layout on 32-bit systems

On Fri, Apr 16, 2021 at 5:27 PM Matthew Wilcox <willy@...radead.org> wrote:
> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> index b5b195305346..db7c7020746a 100644
> --- a/include/net/page_pool.h
> +++ b/include/net/page_pool.h
> @@ -198,7 +198,17 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
>
>  static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
>  {
> -       return page->dma_addr;
> +       dma_addr_t ret = page->dma_addr[0];
> +       if (sizeof(dma_addr_t) > sizeof(unsigned long))
> +               ret |= (dma_addr_t)page->dma_addr[1] << 32;
> +       return ret;
> +}

Have you considered using a PFN type address here? I suspect you
can prove that shifting the DMA address by PAGE_BITS would
make it fit into an 'unsigned long' on all 32-bit architectures with
64-bit dma_addr_t. This requires that page->dma_addr to be
page aligned, as well as fit into 44 bits. I recently went through the
maximum address space per architecture to define a
MAX_POSSIBLE_PHYSMEM_BITS, and none of them have more than
40 here, presumably the same is true for dma address space.

        Arnd