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

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[ Upstream commit f017c1f768 ]

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: 1d2fbaad7c ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Eric Dumazet
2025-09-27 09:28:27 +00:00
committed by Greg Kroah-Hartman
parent f653b133f7
commit f575d6d793
1 changed files with 13 additions and 2 deletions
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15
net/ipv4/tcp_input.c
View File

@@ -4890,12 +4890,23 @@ 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,