netdev - Re: [RFC PATCH] sock: add SO_RCVQUEUE

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Date:   Mon, 13 Mar 2017 12:39:33 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     johunt@...mai.com
Cc:     edumazet@...gle.com, arnd@...db.de, soheil@...gle.com,
        willemb@...gle.com, pabeni@...hat.com, linux-arch@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [RFC PATCH] sock: add SO_RCVQUEUE_SIZE getsockopt

From: Josh Hunt <johunt@...mai.com>
Date: Mon, 13 Mar 2017 12:38:39 -0500

> On 03/13/2017 11:12 AM, Eric Dumazet wrote:
>> On Mon, Mar 13, 2017 at 8:59 AM, Josh Hunt <johunt@...mai.com> wrote:
>>> Allows application to read the amount of data sitting in the receive
>>> queue.
>>>
>>> Signed-off-by: Josh Hunt <johunt@...mai.com>
>>> ---
>>>
>>> A team here is looking for a way to get the amount of data in a UDP
>>> socket's
>>> receive queue. It seems like this should be SIOCINQ, but for UDP
>>> sockets that
>>> returns the size of the next pending datagram. I implemented the patch
>>> below,
>>> but am wondering if this is the right place for this change? I was
>>> debating
>>> between this or a new UDP ioctl.
>>
>> But what is the 'amount of data' exactly ?
>> Number of packets, amount of bytes to read from these packets ?
> 
> I meant bytes. I will clarify in the next version.

As Eric is hinting, the calculation you are using doesn't represent
this.

You need to do something like walk the receive queue and add the
skb->len values together.

sk->sk_rmem_alloc is usually much larger than the sum of the skb->len
values in the socket receive queue.  I don't see how this culmination
of skb->truesize values is useful, whereas I can see how an application
could want the summation of the skb->len values.