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

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Date:	Thu, 22 Dec 2011 19:34:21 +0100
From:	Michał Mirosław <mirqus@...il.com>
To:	Ian Campbell <Ian.Campbell@...rix.com>
Cc:	David Miller <davem@...emloft.net>,
	"eric.dumazet@...il.com" <eric.dumazet@...il.com>,
	"jesse.brandeburg@...el.com" <jesse.brandeburg@...el.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH 0/4] skb paged fragment destructors

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.

Best Regards,
Michał Mirosław
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