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

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Date:	Thu, 22 Dec 2011 13:43:44 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	mirqus@...il.com
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

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.