netdev - Re: [PATCH net-next v3 2/3] udp: implement memory accounting helpers

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Date:   Thu, 29 Sep 2016 07:49:11 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        James Morris <jmorris@...ei.org>,
        Trond Myklebust <trond.myklebust@...marydata.com>,
        Alexander Duyck <aduyck@...antis.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Tom Herbert <tom@...bertland.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Edward Cree <ecree@...arflare.com>, linux-nfs@...r.kernel.org
Subject: Re: [PATCH net-next v3 2/3] udp: implement memory accounting helpers

On Thu, Sep 29, 2016 at 7:34 AM, Paolo Abeni <pabeni@...hat.com> wrote:
> On Thu, 2016-09-29 at 07:13 -0700, Eric Dumazet wrote:
>> On Thu, 2016-09-29 at 16:01 +0200, Paolo Abeni wrote:
>>
>> > When we reach __sk_mem_reduce_allocated() we are sure we can free the
>> > specified amount of memory, so we only need to ensure consistent
>> > sk_prot->memory_allocated updates. The current atomic operation suffices
>> > to this.
>>
>> Then why are you updating sk->sk_forward_alloc using racy operations ?
>>
>> If this is not needed or racy, do not do it.
>
> Thank you for all the feedback.
>
> The actual forward allocated memory value is:
>
> atomic_read(&up->mem_allocated) - atomic_read(&sk->sk_rmem_alloc).
>
> sk_forward_alloc is updated only to hint to the user space the forward
> allocated memory value via the diag interface.
>
> If such information is not needed we can drop the update, and
> sk_forward_alloc will always be seen as 0 even when the socket has some
> forward allocation.

The information is needed and we want an accurate one, really.

Think about debugging on a live server, some stuck or mad sockets ;)

Please consider adding a proper accessor, able to deal with the UDP
peculiarities.


int sk_forward_alloc_get(const struct sock *sk)
{
        if (sk is not UDP)
           return sk->sk_forward_alloc;

       return the precise amount using the private fields that UDP maintains,
}


Then use this accessor in these places :

net/ipv4/af_inet.c:155: WARN_ON(sk->sk_forward_alloc);
net/core/sock_diag.c:66:        mem[SK_MEMINFO_FWD_ALLOC] =
sk->sk_forward_alloc;
net/ipv4/inet_diag.c:191:                       .idiag_fmem =
sk->sk_forward_alloc,
net/sched/em_meta.c:462:        dst->value = sk->sk_forward_alloc;

Thanks.