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

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Message-ID: <CAHXqBFL7oHgeUvKpV3Z2ddhMK-5KEqXJsp7p_geiYWTH_zwtaA@mail.gmail.com>
Date:	Thu, 22 Dec 2011 20:29:30 +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 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?

Best Regards,
Michał Mirosław
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