Re: [PATCH rfc v2 2/5] page_pool: add interface for getting and setting pagecnt_bias

[Alexander Duyck <alexander.duyck@gmail.com>]

On Sat, Jul 10, 2021 at 12:44 AM Yunsheng Lin <linyunsheng@huawei.com> wrote:
>
> As suggested by Alexander, "A DMA mapping should be page
> aligned anyway so the lower 12 bits would be reserved 0",
> so it might make more sense to repurpose the lower 12 bits
> of the dma address to store the pagecnt_bias for elevated
> refcnt case in page pool.
>
> As newly added page_pool_get_pagecnt_bias() may be called
> outside of the softirq context, so annotate the access to
> page->dma_addr[0] with READ_ONCE() and WRITE_ONCE().
>
> Other three interfaces using page->dma_addr[0] is only called
> in the softirq context during normal rx processing, hopefully
> the barrier in the rx processing will ensure the correct order
> between getting and setting pagecnt_bias.
>
> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
> ---
>  include/net/page_pool.h | 24 ++++++++++++++++++++++--
>  1 file changed, 22 insertions(+), 2 deletions(-)
>
> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> index 8d7744d..5746f17 100644
> --- a/include/net/page_pool.h
> +++ b/include/net/page_pool.h
> @@ -200,7 +200,7 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
>
>  static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
>  {
> -       dma_addr_t ret = page->dma_addr[0];
> +       dma_addr_t ret = READ_ONCE(page->dma_addr[0]) & PAGE_MASK;
>         if (sizeof(dma_addr_t) > sizeof(unsigned long))
>                 ret |= (dma_addr_t)page->dma_addr[1] << 16 << 16;
>         return ret;
> @@ -208,11 +208,31 @@ static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
>
>  static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
>  {
> -       page->dma_addr[0] = addr;
> +       unsigned long dma_addr_0 = READ_ONCE(page->dma_addr[0]);
> +
> +       dma_addr_0 &= ~PAGE_MASK;
> +       dma_addr_0 |= (addr & PAGE_MASK);

So rather than doing all this testing and clearing it would probably
be better to add a return value to the function and do something like:

if (WARN_ON(dma_addr_0 & ~PAGE_MASK))
    return -1;

That way you could have page_pool_dma_map unmap, free the page, and
return false indicating that the DMA mapping failed with a visible
error in the event that our expectionat that the dma_addr is page
aligned is ever violated.

> +       WRITE_ONCE(page->dma_addr[0], dma_addr_0);
> +
>         if (sizeof(dma_addr_t) > sizeof(unsigned long))
>                 page->dma_addr[1] = upper_32_bits(addr);
>  }
>
> +static inline int page_pool_get_pagecnt_bias(struct page *page)
> +{
> +       return (READ_ONCE(page->dma_addr[0]) & ~PAGE_MASK);

You don't need the parenthesis around the READ_ONCE and PAGE_MASK.

> +}
> +
> +static inline void page_pool_set_pagecnt_bias(struct page *page, int bias)
> +{
> +       unsigned long dma_addr_0 = READ_ONCE(page->dma_addr[0]);
> +
> +       dma_addr_0 &= PAGE_MASK;
> +       dma_addr_0 |= (bias & ~PAGE_MASK);
> +
> +       WRITE_ONCE(page->dma_addr[0], dma_addr_0);
> +}
> +
>  static inline bool is_page_pool_compiled_in(void)
>  {
>  #ifdef CONFIG_PAGE_POOL
> --
> 2.7.4
>