linux-kernel - Re: [PATCH v4 net-next 1/4] net: core: page_pool: add user cnt preventing pool deletion

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Date:   Tue, 25 Jun 2019 21:36:15 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
Cc:     David Miller <davem@...emloft.net>, grygorii.strashko@...com,
        hawk@...nel.org, brouer@...hat.com, saeedm@...lanox.com,
        leon@...nel.org, Alexei Starovoitov <ast@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-omap@...r.kernel.org, xdp-newbies@...r.kernel.org,
        ilias.apalodimas@...aro.org,
        Network Development <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        jakub.kicinski@...ronome.com,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH v4 net-next 1/4] net: core: page_pool: add user cnt
 preventing pool deletion

On Tue, Jun 25, 2019 at 2:00 PM Ivan Khoronzhuk
<ivan.khoronzhuk@...aro.org> wrote:
>
> Add user counter allowing to delete pool only when no users.
> It doesn't prevent pool from flush, only prevents freeing the
> pool instance. Helps when no need to delete the pool and now
> it's user responsibility to free it by calling page_pool_free()
> while destroying procedure. It also makes to use page_pool_free()
> explicitly, not fully hidden in xdp unreg, which looks more
> correct after page pool "create" routine.
>
> Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
> ---

> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> index f07c518ef8a5..1ec838e9927e 100644
> --- a/include/net/page_pool.h
> +++ b/include/net/page_pool.h
> @@ -101,6 +101,7 @@ struct page_pool {
>         struct ptr_ring ring;
>
>         atomic_t pages_state_release_cnt;
> +       atomic_t user_cnt;

refcount_t?

>  };
>
>  struct page *page_pool_alloc_pages(struct page_pool *pool, gfp_t gfp);
> @@ -183,6 +184,12 @@ static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
>         return page->dma_addr;
>  }
>
> +/* used to prevent pool from deallocation */
> +static inline void page_pool_get(struct page_pool *pool)
> +{
> +       atomic_inc(&pool->user_cnt);
> +}
> +
>  static inline bool is_page_pool_compiled_in(void)
>  {
>  #ifdef CONFIG_PAGE_POOL
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index b366f59885c1..169b0e3c870e 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -48,6 +48,7 @@ static int page_pool_init(struct page_pool *pool,
>                 return -ENOMEM;
>
>         atomic_set(&pool->pages_state_release_cnt, 0);
> +       atomic_set(&pool->user_cnt, 0);
>
>         if (pool->p.flags & PP_FLAG_DMA_MAP)
>                 get_device(pool->p.dev);
> @@ -70,6 +71,8 @@ struct page_pool *page_pool_create(const struct page_pool_params *params)
>                 kfree(pool);
>                 return ERR_PTR(err);
>         }
> +
> +       page_pool_get(pool);
>         return pool;
>  }
>  EXPORT_SYMBOL(page_pool_create);
> @@ -356,6 +359,10 @@ static void __warn_in_flight(struct page_pool *pool)
>
>  void __page_pool_free(struct page_pool *pool)
>  {
> +       /* free only if no users */
> +       if (!atomic_dec_and_test(&pool->user_cnt))
> +               return;
> +
>         WARN(pool->alloc.count, "API usage violation");
>         WARN(!ptr_ring_empty(&pool->ring), "ptr_ring is not empty");
>
> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index 829377cc83db..04bdcd784d2e 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -372,6 +372,9 @@ int xdp_rxq_info_reg_mem_model(struct xdp_rxq_info *xdp_rxq,
>
>         mutex_unlock(&mem_id_lock);
>
> +       if (type == MEM_TYPE_PAGE_POOL)
> +               page_pool_get(xdp_alloc->page_pool);
> +

need an analogous page_pool_put in xdp_rxq_info_unreg_mem_model? mlx5
does not use that inverse function, but intel drivers do.

>         trace_mem_connect(xdp_alloc, xdp_rxq);
>         return 0;
>  err:
> --
> 2.17.1
>