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From: Dust Li <>
To: "David S . Miller" <>,
	Eric Dumazet <>, Hideo Aoki <>
Subject: [PATCH] net/sock: don't drop udp packets if udp_mem[2] not reached
Date: Mon,  7 Sep 2020 22:44:35 +0800	[thread overview]
Message-ID: <> (raw)

We encoutered udp packets drop under a pretty low pressure
with net.ipv4.udp_mem[0] set to a small value (4096).

After some tracing and debugging, we found that for udp
protocol, __sk_mem_raise_allocated() will possiblly drop
packets if:
  udp_mem[0] < udp_prot.memory_allocated < udp_mem[2]

That's because __sk_mem_raise_allocated() didn't handle
the above condition for protocols like udp who doesn't
have sk_has_memory_pressure()

We can reproduce this with the following condition
1. udp_mem[0] is relateive small,
2. net.core.rmem_default/max > udp_mem[0] * 4K
3. The udp server receive slowly, causing the udp_prot->memory_allocated
   exceed udp_mem[0], but still under udp_mem[2]

I wrote a test script to reproduce this:

Obviously, we should not drop packets when udp_prot.memory_allocated
just exceed udp_mem[0] but still under hard limit.

For protocols with memory_pressure callbacks (like TCP), this is
not a problem, because there is an extra check:
  if (sk_has_memory_pressure(sk)) {
  	u64 alloc;

  	if (!sk_under_memory_pressure(sk))
  		return 1;
  	alloc = sk_sockets_allocated_read_positive(sk);
  	if (sk_prot_mem_limits(sk, 2) > alloc *
  			sk_mem_pages(sk->sk_wmem_queued +
  				atomic_read(&sk->sk_rmem_alloc) +
  		return 1;

But UDP didn't check this, so I add an extra check here
to make sure UDP packets are not dropped until the hard limit
is reached.

Signed-off-by: Dust Li <>
 net/core/sock.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/core/sock.c b/net/core/sock.c
index 6c5c6b18eff4..fed8211d8dbe 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -2648,6 +2648,12 @@ int __sk_mem_raise_allocated(struct sock *sk, int size, int amt, int kind)
 				 atomic_read(&sk->sk_rmem_alloc) +
 			return 1;
+	} else {
+		/* for prots without memory_pressure callbacks, we should not
+		 * drop until hard limit reached
+		 */
+		if (allocated <= sk_prot_mem_limits(sk, 2))
+			return 1;

             reply	other threads:[~2020-09-07 15:54 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-07 14:44 Dust Li [this message]
2020-09-07 17:18 ` Paolo Abeni
2020-09-08  3:15   `
2020-09-08  8:46     ` Paolo Abeni
2020-09-09  3:24       `

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