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

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Date:   Thu, 29 Sep 2016 16:59:07 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Eric Dumazet <edumazet@...gle.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, 2016-09-29 at 07:49 -0700, Eric Dumazet wrote:
> 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.

Nice suggestion, thanks! I'll try that in v4. Perhaps is worth adding a
struct proto helper for this ?

I'm sorry, but I'll not be in Tokyo, so I'll probably produce some
traffic on netdev in the meanwhile.

Cheers,

Paolo