netdev - Re: [PATCH net-next v2 2/2] net: ip: avoid OOM kills with large UDP sends over loopback

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Date:   Wed, 23 Jun 2021 08:58:10 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, willemb@...gle.com,
        dsahern@...il.com, yoshfuji@...ux-ipv6.org, Dave Jones <dsj@...com>
Subject: Re: [PATCH net-next v2 2/2] net: ip: avoid OOM kills with large UDP
 sends over loopback

On Wed, 23 Jun 2021 16:25:18 +0200 Eric Dumazet wrote:
> On 6/23/21 12:50 AM, 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.
> > 
> > This is entirely avoidable, we can use an skb with frags.  
> 
> Are you referring to IP fragments, or page frags ?

page frags, annoyingly overloaded term. I'll say paged, it's 
not the common term but at least won't be confusing.

> > af_unix solves a similar problem by limiting the head
> > length to SKB_MAX_ALLOC. This seems like a good and simple
> > approach. It means that UDP messages > 16kB will now
> > use fragments if underlying device supports SG, if extra
> > allocator pressure causes regressions in real workloads
> > we can switch to trying the large allocation first and
> > falling back.
> > 
> > Reported-by: Dave Jones <dsj@...com>
> > Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> > ---
> >  net/ipv4/ip_output.c  | 2 +-
> >  net/ipv6/ip6_output.c | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> > index 90031f5446bd..1ab140c173d0 100644
> > --- a/net/ipv4/ip_output.c
> > +++ b/net/ipv4/ip_output.c
> > @@ -1077,7 +1077,7 @@ static int __ip_append_data(struct sock *sk,
> >  
> >  			if ((flags & MSG_MORE) && !has_sg)
> >  				alloclen = mtu;
> > -			else if (!paged)
> > +			else if (!paged && (fraglen < SKB_MAX_ALLOC || !has_sg))  
> 
> This looks indeed better, but there are some boundary conditions,
> caused by the fact that we add hh_len+15 later when allocating the skb.
> 
> (I expect hh_len+15 being 31)
> 
> 
> You probably need 
> 	else if (!paged && (fraglen + hh_len + 15 < SKB_MAX_ALLOC || !has_sg))
> 
> Otherwise we might still attempt order-3 allocations ?
> 
> SKB_MAX_ALLOC is 16064 currently (skb_shinfo size being 320 on 64bit arches)
> 
> An UDP message with 16034 bytes of payload would translate to
> alloclen==16062. If we add 28 bytes for UDP+IP headers, plus 31 bytes for hh_len+31
> this would go to 16413, thus asking for 32768 bytes (order-3 page)
> 
> (16062+320 = 16382, which is smaller than 16384)

Will do, thanks!

> >  				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 c667b7e2856f..46d805097a79 100644
> > --- a/net/ipv6/ip6_output.c
> > +++ b/net/ipv6/ip6_output.c
> > @@ -1585,7 +1585,7 @@ static int __ip6_append_data(struct sock *sk,
> >  
> >  			if ((flags & MSG_MORE) && !has_sg)
> >  				alloclen = mtu;
> > -			else if (!paged)
> > +			else if (!paged && (fraglen < SKB_MAX_ALLOC || !has_sg))
> >  				alloclen = fraglen;
> >  			else {
> >  				alloclen = min_t(int, fraglen, MAX_HEADER);
> >