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

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Date:	Mon, 21 Apr 2008 19:55:21 -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 12:50:27 +1000

> But I was curious as to why the +2 in the MAX_SKB_FRAGS definition?

To be honest I have no idea.

When Alexey added the TSO changeset way back then, it had the
"+2", from the history-2.6 tree:

commit 80223d5186f73bf42a7e260c66c9cb9f7d8ec9cf
Author: Alexey Kuznetsov <kuznet@....inr.ac.ru>
Date:   Wed Aug 28 11:52:03 2002 -0700

    [NET]: Add TCP segmentation offload core infrastructure.

 ...
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index a812681..9b6e6ad 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -109,7 +109,8 @@ struct sk_buff_head {
 
 struct sk_buff;
 
-#define MAX_SKB_FRAGS 6
+/* To allow 64K frame to be packed as single skb without frag_list */
+#define MAX_SKB_FRAGS (65536/PAGE_SIZE + 2)
 
 typedef struct skb_frag_struct skb_frag_t;
 
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