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tcp: avoid premature drops in tcp_add_backlog()

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While testing TCP performance with latest trees,
I saw suspect SOCKET_BACKLOG drops.

tcp_add_backlog() computes its limit with :

    limit = (u32)READ_ONCE(sk->sk_rcvbuf) +
            (u32)(READ_ONCE(sk->sk_sndbuf) >> 1);
    limit += 64 * 1024;

This does not take into account that sk->sk_backlog.len
is reset only at the very end of __release_sock().

Both sk->sk_backlog.len and sk->sk_rmem_alloc could reach
sk_rcvbuf in normal conditions.

We should double sk->sk_rcvbuf contribution in the formula
to absorb bubbles in the backlog, which happen more often
for very fast flows.

This change maintains decent protection against abuses.

Fixes: c377411f24 ("net: sk_add_backlog() take rmem_alloc into account")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240423125620.3309458-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Eric Dumazet 2024-04-23 12:56:20 +00:00 committed by Jakub Kicinski
parent e6be197f23
commit ec00ed472b
1 changed files with 11 additions and 2 deletions
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13
net/ipv4/tcp_ipv4.c
View file

@ -1995,7 +1995,7 @@ int tcp_v4_early_demux(struct sk_buff *skb)
bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb,
enum skb_drop_reason *reason)
{
u32 limit, tail_gso_size, tail_gso_segs;
u32 tail_gso_size, tail_gso_segs;
struct skb_shared_info *shinfo;
const struct tcphdr *th;
struct tcphdr *thtail;
@ -2004,6 +2004,7 @@ bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb,
bool fragstolen;
u32 gso_segs;
u32 gso_size;
u64 limit;
int delta;
/* In case all data was pulled from skb frags (in __pskb_pull_tail()),
@ -2099,7 +2100,13 @@ bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb,
__skb_push(skb, hdrlen);
no_coalesce:
limit = (u32)READ_ONCE(sk->sk_rcvbuf) + (u32)(READ_ONCE(sk->sk_sndbuf) >> 1);
/* sk->sk_backlog.len is reset only at the end of __release_sock().
* Both sk->sk_backlog.len and sk->sk_rmem_alloc could reach
* sk_rcvbuf in normal conditions.
*/
limit = ((u64)READ_ONCE(sk->sk_rcvbuf)) << 1;
limit += ((u32)READ_ONCE(sk->sk_sndbuf)) >> 1;
/* Only socket owner can try to collapse/prune rx queues
* to reduce memory overhead, so add a little headroom here.
@ -2107,6 +2114,8 @@ bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb,
*/
limit += 64 * 1024;
limit = min_t(u64, limit, UINT_MAX);
if (unlikely(sk_add_backlog(sk, skb, limit))) {
bh_unlock_sock(sk);
*reason = SKB_DROP_REASON_SOCKET_BACKLOG;