netdev - [PATCH 5/5 net-next] net: makes skb_splice_bits() aware of skb->head

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Date:	Fri, 27 Apr 2012 14:10:03 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>,
	Neal Cardwell <ncardwell@...gle.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Tom Herbert <therbert@...gle.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Ben Hutchings <bhutchings@...arflare.com>,
	Matt Carlson <mcarlson@...adcom.com>,
	Michael Chan <mchan@...adcom.com>,
	Maciej Żenczykowski <maze@...gle.com>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Subject: [PATCH 5/5 net-next] net: makes skb_splice_bits() aware of
 skb->head_frag

From: Eric Dumazet <edumazet@...gle.com>

__skb_splice_bits() can check if skb to be spliced has its skb->head
mapped to a page fragment, instead of a kmalloc() area.

If so we can avoid a copy of the skb head and get a reference on
underlying page.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc: Herbert Xu <herbert@...dor.apana.org.au>
Cc: Maciej Żenczykowski <maze@...gle.com>
Cc: Neal Cardwell <ncardwell@...gle.com>
Cc: Tom Herbert <therbert@...gle.com>
Cc: Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Cc: Ben Hutchings <bhutchings@...arflare.com>
Cc: Matt Carlson <mcarlson@...adcom.com>
Cc: Michael Chan <mchan@...adcom.com>
---
 net/core/skbuff.c |   10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 2ad1ee7..e5bd086 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1699,14 +1699,18 @@ static bool __skb_splice_bits(struct sk_buff *skb, struct pipe_inode_info *pipe,
 			      struct splice_pipe_desc *spd, struct sock *sk)
 {
 	int seg;
+	bool head_is_linear = !skb->head_frag;
 
-	/*
-	 * map the linear part
+	/* map the linear part :
+	 * If skb->head_frag is set, this 'linear' part is backed
+	 * by a fragment, and we can avoid a copy.
 	 */
 	if (__splice_segment(virt_to_page(skb->data),
 			     (unsigned long) skb->data & (PAGE_SIZE - 1),
 			     skb_headlen(skb),
-			     offset, len, skb, spd, true, sk, pipe))
+			     offset, len, skb, spd,
+			     head_is_linear,
+			     sk, pipe))
 		return true;
 
 	/*


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