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

[Jesper Dangaard Brouer <brouer@redhat.com>]

On Wed, 23 Jun 2021 09:23:28 -0700
Jakub Kicinski <kuba@kernel.org> 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 page frags.
> 
> 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@fb.com>
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
> ---
>  net/ipv4/ip_output.c  | 4 +++-
>  net/ipv6/ip6_output.c | 4 +++-
>  2 files changed, 6 insertions(+), 2 deletions(-)
> 
> 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?

> +				  !(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);



-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer