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

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Date:   Tue, 22 Jun 2021 19:48:43 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        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] ip: avoid OOM kills with large UDP sends over
 loopback



On 6/22/21 6:54 PM, Jakub Kicinski wrote:
> On Tue, 22 Jun 2021 16:12:11 +0200 Eric Dumazet wrote:
>> 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>
> 
>>> +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.
> 
> Ugh, you're right :(
> 
>>> +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 ?
> 
> PAGE_ALLOC_COSTLY_ORDER in itself is more of a problem than a solution.
> AFAIU the app sends messages primarily above the ~60kB mark, which is
> above COSTLY, and those do not trigger OOM kills. All OOM kills we see
> have order=3. Checking with Rik and Johannes W that's expected, OOM
> killer is only invoked for allocations <= COSTLY, larger ones will just
> return NULL and let us deal with it (e.g. by falling back).

I  really thought alloc_skb_with_frags() was already handling low-memory-conditions.

(alloc_skb_with_frags() is called from sock_alloc_send_pskb())

If it is not, lets fix it, because af_unix sockets will have the same issue ?



> 
> So adding GFP_NORETRY is key for 0 < order <= COSTLY,
> skb_page_frag_refill()-style.
> 
>> 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)