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

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Message-ID: <8fe00e04-3a79-6439-6ec7-5e40408529e2@gmail.com>
Date:   Tue, 22 Jun 2021 16:12:11 +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] ip: avoid OOM kills with large UDP sends over
 loopback



On 6/22/21 1:13 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.
> 
> The scenario is unlikely and always using frags requires
> an extra allocation so opt for using fallback, rather
> then always using frag'ed/paged skb when payload is large.
> 
> Note that the size heuristic (header_len > PAGE_SIZE)
> is not entirely accurate, __alloc_skb() will add ~400B
> to size. Occasional order-1 allocation should be fine,
> though, we are primarily concerned with order-3.
> 
> Reported-by: Dave Jones <dsj@...com>
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> ---
>  include/net/sock.h    | 11 +++++++++++
>  net/ipv4/ip_output.c  | 19 +++++++++++++++++--
>  net/ipv6/ip6_output.c | 19 +++++++++++++++++--
>  3 files changed, 45 insertions(+), 4 deletions(-)
> 
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 7a7058f4f265..4134fb718b97 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -924,6 +924,17 @@ static inline gfp_t sk_gfp_mask(const struct sock *sk, gfp_t gfp_mask)
>  	return gfp_mask | (sk->sk_allocation & __GFP_MEMALLOC);
>  }
>  
> +static inline void sk_allocation_push(struct sock *sk, gfp_t flag, gfp_t *old)
> +{
> +	*old = sk->sk_allocation;
> +	sk->sk_allocation |= flag;
> +}
> +

This is not thread safe.

Remember UDP sendmsg() does not lock the socket for non-corking sends.

> +static inline void sk_allocation_pop(struct sock *sk, gfp_t old)
> +{
> +	sk->sk_allocation = old;
> +}
> +
>  static inline void sk_acceptq_removed(struct sock *sk)
>  {
>  	WRITE_ONCE(sk->sk_ack_backlog, sk->sk_ack_backlog - 1);
> 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);
> +				skb = sock_alloc_send_skb(sk, header_len,
>  						(flags & MSG_DONTWAIT), &err);
> +				if (header_len > PAGE_SIZE) {
> +					BUILD_BUG_ON(MAX_HEADER >= PAGE_SIZE);
> +
> +					sk_allocation_pop(sk, sk_allocation);
> +					if (unlikely(!skb) && !paged &&
> +					    rt->dst.dev->features & NETIF_F_SG) {
> +						paged = true;
> +						goto alloc_new_skb;
> +					}
> +				}


What about using sock_alloc_send_pskb(... PAGE_ALLOC_COSTLY_ORDER)
(as we did in unix_dgram_sendmsg() for large packets), for SG enabled interfaces ?

We do not _have_ to put all the payload in skb linear part,
we could instead use page frags (order-0 if high order pages are not available)


>  			} else {
>  				skb = NULL;
>  				if (refcount_read(&sk->sk_wmem_alloc) + wmem_alloc_delta <=