tcp: use skb->len instead of skb->truesize in tcp_can_ingest()

Some applications are stuck to the 20th century and still use
small SO_RCVBUF values.

After the blamed commit, we can drop packets especially
when using LRO/hw-gro enabled NIC and small MSS (1500) values.

LRO/hw-gro NIC pack multiple segments into pages, allowing
tp->scaling_ratio to be set to a high value.

Whenever the receive queue gets full, we can receive a small packet
filling RWIN, but with a high skb->truesize, because most NIC use 4K page
plus sk_buff metadata even when receiving less than 1500 bytes of payload.

Even if we refine how tp->scaling_ratio is estimated,
we could have an issue at the start of the flow, because
the first round of packets (IW10) will be sent based on
the initial tp->scaling_ratio (1/2)

Relax tcp_can_ingest() to use skb->len instead of skb->truesize,
allowing the peer to use final RWIN, assuming a 'perfect'
scaling_ratio of 1.

Fixes: 1d2fbaa ("tcp: stronger sk_rcvbuf checks")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250927092827.2707901-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/ipv4/tcp_input.c

@@ -5086,12 +5086,23 @@ static int tcp_prune_queue(struct sock *sk, const struct sk_buff *in_skb);
 
 /* Check if this incoming skb can be added to socket receive queues
  * while satisfying sk->sk_rcvbuf limit.
+ *
+ * In theory we should use skb->truesize, but this can cause problems
+ * when applications use too small SO_RCVBUF values.
+ * When LRO / hw gro is used, the socket might have a high tp->scaling_ratio,
+ * allowing RWIN to be close to available space.
+ * Whenever the receive queue gets full, we can receive a small packet
+ * filling RWIN, but with a high skb->truesize, because most NIC use 4K page
+ * plus sk_buff metadata even when receiving less than 1500 bytes of payload.
+ *
+ * Note that we use skb->len to decide to accept or drop this packet,
+ * but sk->sk_rmem_alloc is the sum of all skb->truesize.
  */
 static bool tcp_can_ingest(const struct sock *sk, const struct sk_buff *skb)
 {
-	unsigned int new_mem = atomic_read(&sk->sk_rmem_alloc) + skb->truesize;
+	unsigned int rmem = atomic_read(&sk->sk_rmem_alloc);
 
-	return new_mem <= sk->sk_rcvbuf;
+	return rmem + skb->len <= sk->sk_rcvbuf;
 }
 
 static int tcp_try_rmem_schedule(struct sock *sk, const struct sk_buff *skb,