Re: [PATCH bpf-next v3 3/4] xdp: recycle Page Pool backed skbs built from XDP frames

[Jesper Dangaard Brouer]

On 13/03/2023 22.55, Alexander Lobakin wrote:
> __xdp_build_skb_from_frame() state(d):
>
> /* Until page_pool get SKB return path, release DMA here */
>
> Page Pool got skb pages recycling in April 2021, but missed this
> function.
>
> xdp_release_frame() is relevant only for Page Pool backed frames and it
> detaches the page from the corresponding page_pool in order to make it
> freeable via page_frag_free(). It can instead just mark the output skb
> as eligible for recycling if the frame is backed by a pp. No change for
> other memory model types (the same condition check as before).
> cpumap redirect and veth on Page Pool drivers now become zero-alloc (or
> almost).
>
> Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
> ---
>   net/core/xdp.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index 8c92fc553317..a2237cfca8e9 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -658,8 +658,8 @@ struct sk_buff *__xdp_build_skb_from_frame(struct xdp_frame *xdpf,
>   	 * - RX ring dev queue index	(skb_record_rx_queue)
>   	 */
>   
> -	/* Until page_pool get SKB return path, release DMA here */
> -	xdp_release_frame(xdpf);
> +	if (xdpf->mem.type == MEM_TYPE_PAGE_POOL)
> +		skb_mark_for_recycle(skb);

I hope this is safe ;-) ... Meaning hopefully drivers does the correct
thing when XDP_REDIRECT'ing page_pool pages.

Looking for drivers doing weird refcnt tricks and XDP_REDIRECT'ing, I
noticed the driver aquantia/atlantic (in aq_get_rxpages_xdp), but I now
see this is not using page_pool, so it should be affected by this (but I
worry if atlantic driver have a potential race condition for its refcnt
scheme).

>   
>   	/* Allow SKB to reuse area used by xdp_frame */
>   	xdp_scrub_frame(xdpf);