Re: [PATCH net-next 1/2] page_pool: support non-split page with PP_FLAG_PAGE_FRAG

[Alexander Duyck]

On Sun, Aug 29, 2021 at 6:19 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
>
> Currently when PP_FLAG_PAGE_FRAG is set, the caller is not
> expected to call page_pool_alloc_pages() directly because of
> the PP_FLAG_PAGE_FRAG checking in __page_pool_put_page().
>
> The patch removes the above checking to enable non-split page
> support when PP_FLAG_PAGE_FRAG is set.
>
> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
> ---
>  include/net/page_pool.h |  6 ++++++
>  net/core/page_pool.c    | 12 +++++++-----
>  2 files changed, 13 insertions(+), 5 deletions(-)
>
> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> index a408240..2ad0706 100644
> --- a/include/net/page_pool.h
> +++ b/include/net/page_pool.h
> @@ -238,6 +238,9 @@ static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
>
>  static inline void page_pool_set_frag_count(struct page *page, long nr)
>  {
> +       if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
> +               return;
> +
>         atomic_long_set(&page->pp_frag_count, nr);
>  }
>
> @@ -246,6 +249,9 @@ static inline long page_pool_atomic_sub_frag_count_return(struct page *page,
>  {
>         long ret;
>
> +       if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
> +               return 0;
> +
>         /* As suggested by Alexander, atomic_long_read() may cover up the
>          * reference count errors, so avoid calling atomic_long_read() in
>          * the cases of freeing or draining the page_frags, where we would
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index 1a69784..ba9f14d 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -313,11 +313,14 @@ struct page *page_pool_alloc_pages(struct page_pool *pool, gfp_t gfp)
>
>         /* Fast-path: Get a page from cache */
>         page = __page_pool_get_cached(pool);
> -       if (page)
> -               return page;
>
>         /* Slow-path: cache empty, do real allocation */
> -       page = __page_pool_alloc_pages_slow(pool, gfp);
> +       if (!page)
> +               page = __page_pool_alloc_pages_slow(pool, gfp);
> +
> +       if (likely(page))
> +               page_pool_set_frag_count(page, 1);
> +
>         return page;
>  }
>  EXPORT_SYMBOL(page_pool_alloc_pages);
> @@ -426,8 +429,7 @@ __page_pool_put_page(struct page_pool *pool, struct page *page,
>                      unsigned int dma_sync_size, bool allow_direct)
>  {
>         /* It is not the last user for the page frag case */
> -       if (pool->p.flags & PP_FLAG_PAGE_FRAG &&
> -           page_pool_atomic_sub_frag_count_return(page, 1))
> +       if (page_pool_atomic_sub_frag_count_return(page, 1))
>                 return NULL;

Isn't this going to have a negative performance impact on page pool
pages in general? Essentially you are adding an extra atomic operation
for all the non-frag pages.

It would work better if this was doing a check against 1 to determine
if it is okay for this page to be freed here and only if the check
fails then you perform the atomic sub_return.