Re: [PATCH net-next] ip: avoid OOM kills with large UDP sends over loopback

On Tue, 22 Jun 2021 12:07:27 +0200 Paolo Abeni wrote:

On Mon, 2021-06-21 at 16:13 -0700, Jakub Kicinski wrote:

quoted

Dave observed number of machines hitting OOM on the UDP send
path. The workload seems to be sending large UDP packets over
loopback. Since loopback has MTU of 64k kernel will try to
allocate an skb with up to 64k of head space. This has a good
chance of failing under memory pressure. What's worse if
the message length is <32k the allocation may trigger an
OOM killer.  

Out of sheer curiosity, are there a large number of UDP sockets in such
workload? did you increase rmem_default/rmem_max? If so, could tuning
udp_mem help?

It's a handful of sockets, < 10.

quoted

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index c3efc7d658f6..a300c2c65d57 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c

@@ -1095,9 +1095,24 @@ static int __ip_append_data(struct sock *sk,
 				alloclen += rt->dst.trailer_len;
 
 			if (transhdrlen) {
-				skb = sock_alloc_send_skb(sk,
-						alloclen + hh_len + 15,
+				size_t header_len = alloclen + hh_len + 15;
+				gfp_t sk_allocation;
+
+				if (header_len > PAGE_SIZE)
+					sk_allocation_push(sk, __GFP_NORETRY,
+							   &sk_allocation);  

Could an additional __GFP_NOWARN be relevant here?

We always set GFP_NOWARN for heads thru kmalloc_reserve().