netdev - Re: [PATCH net-next 1/7] page_pool: disable dma mapping support for 32-bit arch with 64-bit DMA

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Message-ID: <CAC_iWjJX1N_Hi42muo=FUD6oNapwo7mi3af5Gj7=24XSJJbYtQ@mail.gmail.com>
Date:   Thu, 23 Sep 2021 12:10:09 +0300
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Networking <netdev@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        linuxarm@...neuler.org, Jesper Dangaard Brouer <hawk@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Alexander Lobakin <alobakin@...me>,
        Willem de Bruijn <willemb@...gle.com>,
        Cong Wang <cong.wang@...edance.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Kevin Hao <haokexin@...il.com>,
        Aleksandr Nogikh <nogikh@...gle.com>,
        Marco Elver <elver@...gle.com>, memxor@...il.com,
        Eric Dumazet <edumazet@...gle.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        David Ahern <dsahern@...il.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH net-next 1/7] page_pool: disable dma mapping support for
 32-bit arch with 64-bit DMA

(+cc Matthew) but this looks safe to me.

On Wed, 22 Sept 2021 at 12:43, Yunsheng Lin <linyunsheng@...wei.com> wrote:
>
> As the 32-bit arch with 64-bit DMA seems to rare those days,
> and page pool is carrying a lot of code and complexity for
> systems that possibly don't exist.
>
> So disable dma mapping support for such systems, if drivers
> really want to work on such systems, they have to implement
> their own DMA-mapping fallback tracking outside page_pool.
>
> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
> ---
>  include/linux/mm_types.h | 13 +------------
>  include/net/page_pool.h  | 12 +-----------
>  net/core/page_pool.c     | 10 ++++++----
>  3 files changed, 8 insertions(+), 27 deletions(-)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 7f8ee09c711f..436e0946d691 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -104,18 +104,7 @@ struct page {
>                         struct page_pool *pp;
>                         unsigned long _pp_mapping_pad;
>                         unsigned long dma_addr;
> -                       union {
> -                               /**
> -                                * dma_addr_upper: might require a 64-bit
> -                                * value on 32-bit architectures.
> -                                */
> -                               unsigned long dma_addr_upper;
> -                               /**
> -                                * For frag page support, not supported in
> -                                * 32-bit architectures with 64-bit DMA.
> -                                */
> -                               atomic_long_t pp_frag_count;
> -                       };
> +                       atomic_long_t pp_frag_count;
>                 };
>                 struct {        /* slab, slob and slub */
>                         union {
> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> index a4082406a003..3855f069627f 100644
> --- a/include/net/page_pool.h
> +++ b/include/net/page_pool.h
> @@ -216,24 +216,14 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
>         page_pool_put_full_page(pool, page, true);
>  }
>
> -#define PAGE_POOL_DMA_USE_PP_FRAG_COUNT        \
> -               (sizeof(dma_addr_t) > sizeof(unsigned long))
> -
>  static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
>  {
> -       dma_addr_t ret = page->dma_addr;
> -
> -       if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
> -               ret |= (dma_addr_t)page->dma_addr_upper << 16 << 16;
> -
> -       return ret;
> +       return page->dma_addr;
>  }
>
>  static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
>  {
>         page->dma_addr = addr;
> -       if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
> -               page->dma_addr_upper = upper_32_bits(addr);
>  }
>
>  static inline void page_pool_set_frag_count(struct page *page, long nr)
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index 1a6978427d6c..a65bd7972e37 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -49,6 +49,12 @@ static int page_pool_init(struct page_pool *pool,
>          * which is the XDP_TX use-case.
>          */
>         if (pool->p.flags & PP_FLAG_DMA_MAP) {
> +               /* DMA-mapping is not supported on 32-bit systems with
> +                * 64-bit DMA mapping.
> +                */
> +               if (sizeof(dma_addr_t) > sizeof(unsigned long))
> +                       return -EINVAL;
> +
>                 if ((pool->p.dma_dir != DMA_FROM_DEVICE) &&
>                     (pool->p.dma_dir != DMA_BIDIRECTIONAL))
>                         return -EINVAL;
> @@ -69,10 +75,6 @@ static int page_pool_init(struct page_pool *pool,
>                  */
>         }
>
> -       if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT &&
> -           pool->p.flags & PP_FLAG_PAGE_FRAG)
> -               return -EINVAL;
> -
>         if (ptr_ring_init(&pool->ring, ring_qsize, GFP_KERNEL) < 0)
>                 return -ENOMEM;
>
> --
> 2.33.0
>

Reviewed-by: Ilias Apalodimas <ilias.apalodimas@...aro.org>