Re: [PATCH v3] page_pool: add a comment explaining the fragment counter usage

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

On 17/02/2023 23.21, Ilias Apalodimas wrote:
> When reading the page_pool code the first impression is that keeping
> two separate counters, one being the page refcnt and the other being
> fragment pp_frag_count, is counter-intuitive.
> 
> However without that fragment counter we don't know when to reliably
> destroy or sync the outstanding DMA mappings.  So let's add a comment
> explaining this part.
> 
> Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>


> ---
> Changes since v2:
>   - Removed a uneeded commas on the comment
> Changes since v1:
>   - Update the comment withe the correct description for pp_frag_count
>   include/net/page_pool.h | 10 ++++++++++
>   1 file changed, 10 insertions(+)
> 
> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> index 34bf531ffc8d..ddfa0b328677 100644
> --- a/include/net/page_pool.h
> +++ b/include/net/page_pool.h
> @@ -277,6 +277,16 @@ void page_pool_put_defragged_page(struct page_pool *pool, struct page *page,
>   				  unsigned int dma_sync_size,
>   				  bool allow_direct);
> 
> +/* pp_frag_count represents the number of writers who can update the page
> + * either by updating skb->data or via DMA mappings for the device.
> + * We can't rely on the page refcnt for that as we don't know who might be
> + * holding page references and we can't reliably destroy or sync DMA mappings
> + * of the fragments.
> + *
> + * When pp_frag_count reaches 0 we can either recycle the page if the page
> + * refcnt is 1 or return it back to the memory allocator and destroy any
> + * mappings we have.
> + */
>   static inline void page_pool_fragment_page(struct page *page, long nr)
>   {
>   	atomic_long_set(&page->pp_frag_count, nr);
> --
> 2.38.1
>