netdev - Re: [PATCH 0/4] skb paged fragment destructors

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Date:	Fri, 23 Dec 2011 19:10:23 +0100
From:	Michał Mirosław <mirqus@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	Ian.Campbell@...rix.com, eric.dumazet@...il.com,
	jesse.brandeburg@...el.com, netdev@...r.kernel.org
Subject: Re: [PATCH 0/4] skb paged fragment destructors

2011/12/22 David Miller <davem@...emloft.net>:
> From: Michał Mirosław <mirqus@...il.com>
> Date: Thu, 22 Dec 2011 20:29:30 +0100
>
>> 2011/12/22 David Miller <davem@...emloft.net>:
>>> From: Michał Mirosław <mirqus@...il.com>
>>> Date: Thu, 22 Dec 2011 19:34:21 +0100
>>>
>>>> 2011/12/22 Ian Campbell <Ian.Campbell@...rix.com>:
>>>>> On Wed, 2011-12-21 at 19:28 +0000, David Miller wrote:
>>>>>> From: Eric Dumazet <eric.dumazet@...il.com>
>>>>>> Date: Wed, 21 Dec 2011 15:02:18 +0100
>>>>>> > No idea on this +2 point.
>>>>>> I think I know, and I believe I instructed Alexey Kuznetsov to do
>>>>>> this.
>>>>>>
>>>>>> When sendfile() is performed, we might start the SKB with the last few
>>>>>> bytes of one page, and end the SKB with the first few bytes of another
>>>>>> page.
>>>>>>
>>>>>> In order to fit a full 64K frame into an SKB in this situation we have
>>>>>> to accomodate this case.
>>>>> Thanks David, that makes sense.
>>>>>
>>>>> However I think you only actually need 1 extra page for that. If the
>>>>> data in frag[0] starts at $offset then frag[16] will need to have
>>>>> $offset bytes in it. e.g.
>>>>>        4096-$offset + 4096*15 + $offset = 65536
>>>>> which == 17 pages rather than 18.
>>>>>
>>>>> The following documents the status quo but I could update to switch to +
>>>>> 1 instead if there are no flaws in the above logic...
>>>>
>>>> Since max IP datagram is 64K-1, adding ethernet and possibly VLAN
>>>> headers makes the max size slightly above 64K and then you have
>>>> 64K/PAGE_SIZE+2 pages appear in worst case.
>>>
>>> Headers go into the linear area, so they are not relevant for these
>>> calculations.
>>
>> Does this hold for LRO'ed packets and packets sent via PF_PACKET socket?
>
> LRO is for receive, not packets we build on transmit, and in any event
> have their headers also pulled into the linear area before the stack
> sees it.

Yes, but the drivers seem to first fill the data to frags[] and only
then move the packets header (I browsed myri10ge).

> For PF_PACKET, in the packet_snd() case it uses a linear SKB and in
> the tpacket_snd() case it uses a linear SKB as well.

I saw skb_fill_page_desc() in af_packet.c (called by
tpacket_fill_skb() which is called by tpacket_snd()), that's why I'm
asking.

Best Regards,
Michał Mirosław
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