netdev - Re: [PATCH net-next v3] net: ip: avoid OOM kills with large UDP sends over loopback

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Date:   Wed, 23 Jun 2021 14:07:17 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Jesper Dangaard Brouer <brouer@...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 v3] net: ip: avoid OOM kills with large UDP
 sends over loopback

On Wed, 23 Jun 2021 21:45:55 +0200 Jesper Dangaard Brouer wrote:
> > diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> > index c3efc7d658f6..790dd28fd198 100644
> > --- a/net/ipv4/ip_output.c
> > +++ b/net/ipv4/ip_output.c
> > @@ -1077,7 +1077,9 @@ static int __ip_append_data(struct sock *sk,
> >  			if ((flags & MSG_MORE) &&
> >  			    !(rt->dst.dev->features&NETIF_F_SG))
> >  				alloclen = mtu;
> > -			else if (!paged)
> > +			else if (!paged &&
> > +				 (fraglen + hh_len + 15 < SKB_MAX_ALLOC ||  
> 
> What does the number 15 represent here?

No idea, it's there on the allocation line, so I need to include it on
the size check.

Looking at super old code (2.4.x) it looks like it may have gotten
copy & pasted mistakenly? The hard headers are rounded up to 16B,
and there is code which does things like:

	skb_alloc(size + dev->hard_header_len + 15);
	skb_reserve(skb, (dev->hard_header_len + 15) & ~15);

in other spots. So if I was to guess I'd say someone decided to add the
15B "to be safe" even though hh_len already includes the round up here.

But that's just my guess. I can't get this simple patch right,
so take that with a grain of salt :/

> > +				  !(rt->dst.dev->features & NETIF_F_SG)))
> >  				alloclen = fraglen;
> >  			else {
> >  				alloclen = min_t(int, fraglen, MAX_HEADER);
> > diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> > index ff4f9ebcf7f6..ae8dbd6cdab1 100644
> > --- a/net/ipv6/ip6_output.c
> > +++ b/net/ipv6/ip6_output.c
> > @@ -1585,7 +1585,9 @@ static int __ip6_append_data(struct sock *sk,
> >  			if ((flags & MSG_MORE) &&
> >  			    !(rt->dst.dev->features&NETIF_F_SG))
> >  				alloclen = mtu;
> > -			else if (!paged)
> > +			else if (!paged &&
> > +				 (fraglen + hh_len < SKB_MAX_ALLOC ||  
> 
> The number 15 is not use here.
> 
> > +				  !(rt->dst.dev->features & NETIF_F_SG)))
> >  				alloclen = fraglen;
> >  			else {
> >  				alloclen = min_t(int, fraglen, MAX_HEADER);