xdp: improve page_pool xdp_return performance

During LPC2022 I meetup with my page_pool co-maintainer Ilias. When
discussing page_pool code we realised/remembered certain optimizations
had not been fully utilised.

Since commit c07aea3e ("mm: add a signature in struct page") struct
page have a direct pointer to the page_pool object this page was
allocated from.

Thus, with this info it is possible to skip the rhashtable_lookup to
find the page_pool object in __xdp_return().

The rcu_read_lock can be removed as it was tied to xdp_mem_allocator.
The page_pool object is still safe to access as it tracks inflight pages
and (potentially) schedules final release from a work queue.

Created a micro benchmark of XDP redirecting from mlx5 into veth with
XDP_DROP bpf-prog on the peer veth device. This increased performance
6.5% from approx 8.45Mpps to 9Mpps corresponding to using 7 nanosec
(27 cycles at 3.8GHz) less per packet.
Suggested-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Link: https://lore.kernel.org/r/166377993287.1737053.10258297257583703949.stgit@firesoulSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/xdp.c

@@ -375,19 +375,17 @@ EXPORT_SYMBOL_GPL(xdp_rxq_info_reg_mem_model);
 void __xdp_return(void *data, struct xdp_mem_info *mem, bool napi_direct,
 		  struct xdp_buff *xdp)
 {
-	struct xdp_mem_allocator *xa;
 	struct page *page;
 
 	switch (mem->type) {
 	case MEM_TYPE_PAGE_POOL:
-		rcu_read_lock();
-		/* mem->id is valid, checked in xdp_rxq_info_reg_mem_model() */
-		xa = rhashtable_lookup(mem_id_ht, &mem->id, mem_id_rht_params);
 		page = virt_to_head_page(data);
 		if (napi_direct && xdp_return_frame_no_direct())
 			napi_direct = false;
-		page_pool_put_full_page(xa->page_pool, page, napi_direct);
-		rcu_read_unlock();
+		/* No need to check ((page->pp_magic & ~0x3UL) == PP_SIGNATURE)
+		 * as mem->type knows this a page_pool page
+		 */
+		page_pool_put_full_page(page->pp, page, napi_direct);
 		break;
 	case MEM_TYPE_PAGE_SHARED:
 		page_frag_free(data);