netdev - Re: [6/6] [VIRTIO] net: Allow receiving SG packets

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Date:	Mon, 21 Apr 2008 13:04:18 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	rusty@...tcorp.com.au
Cc:	virtualization@...ts.linux-foundation.org,
	herbert@...dor.apana.org.au, virtualization@...ts.osdl.org,
	netdev@...r.kernel.org
Subject: Re: [6/6] [VIRTIO] net: Allow receiving SG packets

From: Rusty Russell <rusty@...tcorp.com.au>
Date: Tue, 22 Apr 2008 05:06:16 +1000

> I'm not sure what the right number is here.  Say worst case is header which 
> goes over a page boundary then MAX_SKB_FRAGS in the skb, but for some reason 
> that already has a +2:
> 
> /* To allow 64K frame to be packed as single skb without frag_list */
> #define MAX_SKB_FRAGS (65536/PAGE_SIZE + 2)
> 
> Unless someone explains, I'll change the xmit sg to 2+MAX_SKB_FRAGS as well.

MAX_SKB_FRAGS + 1 is what you ought to need.

MAX_SKB_FRAGS is only accounting for the skb frag pages.
If you want to know how many segments skb->data might
consume as well, you have to add one.

skb->data is linear, therefore it's not possible to need
more than one scatterlist entry for it.
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