netdev - [PATCH v2 bpf-next 1/3] xsk: avoid overwriting skb fields for multi-buffer traffic

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Message-Id: <20250925160009.2474816-2-maciej.fijalkowski@intel.com>
Date: Thu, 25 Sep 2025 18:00:07 +0200
From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To: bpf@...r.kernel.org,
	ast@...nel.org,
	daniel@...earbox.net,
	andrii@...nel.org
Cc: netdev@...r.kernel.org,
	magnus.karlsson@...el.com,
	stfomichev@...il.com,
	kerneljasonxing@...il.com,
	Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Subject: [PATCH v2 bpf-next 1/3] xsk: avoid overwriting skb fields for multi-buffer traffic

We are unnecessarily setting a bunch of skb fields per each processed
descriptor, which is redundant for fragmented frames.

Let us set these respective members for first fragment only. To address
both paths that we have within xsk_build_skb(), move assignments onto
xsk_set_destructor_arg() and rename it to xsk_skb_init_misc().

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
---
 net/xdp/xsk.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 72e34bd2d925..01f258894fae 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -618,11 +618,16 @@ static void xsk_destruct_skb(struct sk_buff *skb)
 	sock_wfree(skb);
 }
 
-static void xsk_set_destructor_arg(struct sk_buff *skb, u64 addr)
+static void xsk_skb_init_misc(struct sk_buff *skb, struct xdp_sock *xs,
+			      u64 addr)
 {
 	BUILD_BUG_ON(sizeof(struct xsk_addr_head) > sizeof(skb->cb));
 	INIT_LIST_HEAD(&XSKCB(skb)->addrs_list);
+	skb->dev = xs->dev;
+	skb->priority = READ_ONCE(xs->sk.sk_priority);
+	skb->mark = READ_ONCE(xs->sk.sk_mark);
 	XSKCB(skb)->num_descs = 0;
+	skb->destructor = xsk_destruct_skb;
 	skb_shinfo(skb)->destructor_arg = (void *)(uintptr_t)addr;
 }
 
@@ -673,7 +678,7 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
 
 		skb_reserve(skb, hr);
 
-		xsk_set_destructor_arg(skb, desc->addr);
+		xsk_skb_init_misc(skb, xs, desc->addr);
 	} else {
 		xsk_addr = kmem_cache_zalloc(xsk_tx_generic_cache, GFP_KERNEL);
 		if (!xsk_addr)
@@ -757,7 +762,7 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
 			if (unlikely(err))
 				goto free_err;
 
-			xsk_set_destructor_arg(skb, desc->addr);
+			xsk_skb_init_misc(skb, xs, desc->addr);
 		} else {
 			int nr_frags = skb_shinfo(skb)->nr_frags;
 			struct xsk_addr_node *xsk_addr;
@@ -826,14 +831,10 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
 
 			if (meta->flags & XDP_TXMD_FLAGS_LAUNCH_TIME)
 				skb->skb_mstamp_ns = meta->request.launch_time;
+			xsk_tx_metadata_to_compl(meta, &skb_shinfo(skb)->xsk_meta);
 		}
 	}
 
-	skb->dev = dev;
-	skb->priority = READ_ONCE(xs->sk.sk_priority);
-	skb->mark = READ_ONCE(xs->sk.sk_mark);
-	skb->destructor = xsk_destruct_skb;
-	xsk_tx_metadata_to_compl(meta, &skb_shinfo(skb)->xsk_meta);
 	xsk_inc_num_desc(skb);
 
 	return skb;
-- 
2.43.0