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

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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);




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