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

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Date:   Tue, 22 Jun 2021 09:57:25 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     davem@...emloft.net, 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 Tue, 22 Jun 2021 12:07:27 +0200 Paolo Abeni wrote:
> 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?

It's a handful of sockets, < 10.

> > 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().