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

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Message-ID: <11f88247-3193-bae4-39e5-03a5672578f9@gmail.com>
Date:   Wed, 23 Jun 2021 16:25:18 +0200
From:   Eric Dumazet <eric.dumazet@...il.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 v2 2/2] net: ip: avoid OOM kills with large UDP
 sends over loopback



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 ?

> 
> 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)


>  				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);
>