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

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Date:   Tue, 22 Jun 2021 12:07:27 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>, davem@...emloft.net
Cc:     netdev@...r.kernel.org, willemb@...gle.com, eric.dumazet@...il.com,
        dsahern@...il.com, yoshfuji@...ux-ipv6.org, Dave Jones <dsj@...com>
Subject: Re: [PATCH net-next] ip: avoid OOM kills with large UDP sends over
 loopback

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

> include/net/sock.h    | 11 +++++++++++
>  net/ipv4/ip_output.c  | 19 +++++++++++++++++--
>  net/ipv6/ip6_output.c | 19 +++++++++++++++++--
>  3 files changed, 45 insertions(+), 4 deletions(-)
> 
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 7a7058f4f265..4134fb718b97 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -924,6 +924,17 @@ static inline gfp_t sk_gfp_mask(const struct sock *sk, gfp_t gfp_mask)
>  	return gfp_mask | (sk->sk_allocation & __GFP_MEMALLOC);
>  }
>  
> +static inline void sk_allocation_push(struct sock *sk, gfp_t flag, gfp_t *old)
> +{
> +	*old = sk->sk_allocation;
> +	sk->sk_allocation |= flag;
> +}
> +
> +static inline void sk_allocation_pop(struct sock *sk, gfp_t old)
> +{
> +	sk->sk_allocation = old;
> +}
> +
>  static inline void sk_acceptq_removed(struct sock *sk)
>  {
>  	WRITE_ONCE(sk->sk_ack_backlog, sk->sk_ack_backlog - 1);
> 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?

Thanks!

Paolo