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

[Arnd Bergmann <arnd@kernel.org>]

On Fri, Apr 16, 2021 at 5:27 PM Matthew Wilcox <willy@infradead.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