netdev - [PATCH v3] net: don't reallocate skb->head unless the current one hasn't the needed extra size or is shared

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Date:	Tue, 30 Nov 2010 16:48:46 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	"David S. Miller" <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, Changli Gao <xiaosuo@...il.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH v3] net: don't reallocate skb->head unless the current one hasn't the needed extra size or is shared

skb head being allocated by kmalloc(), it might be larger than what
actually requested because of discrete kmem caches sizes. Before
reallocating a new skb head, check if the current one has the needed
extra size.

Do this check only if skb head is not shared.

Signed-off-by: Changli Gao <xiaosuo@...il.com>
Cc: Eric Dumazet <eric.dumazet@...il.com>
---
V3: update the comment.
v2: apply this trick for the cloned but not shared skb
 net/core/skbuff.c |   34 +++++++++++++++++++++++-----------
 1 file changed, 23 insertions(+), 11 deletions(-)
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 104f844..8814a9a 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -778,6 +778,28 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
 
 	size = SKB_DATA_ALIGN(size);
 
+	/* Check if we can avoid taking references on fragments if we own
+	 * the last reference on skb->head. (see skb_release_data())
+	 */
+	if (!skb->cloned)
+		fastpath = true;
+	else {
+		int delta = skb->nohdr ? (1 << SKB_DATAREF_SHIFT) + 1 : 1;
+
+		fastpath = atomic_read(&skb_shinfo(skb)->dataref) == delta;
+	}
+
+	if (fastpath &&
+	    size + sizeof(struct skb_shared_info) <= ksize(skb->head)) {
+		memmove(skb->head + size, skb_shinfo(skb),
+			offsetof(struct skb_shared_info,
+				 frags[skb_shinfo(skb)->nr_frags]));
+		memmove(skb->head + nhead, skb->head,
+			skb_tail_pointer(skb) - skb->head);
+		off = nhead;
+		goto adjust_others;
+	}
+
 	data = kmalloc(size + sizeof(struct skb_shared_info), gfp_mask);
 	if (!data)
 		goto nodata;
@@ -791,17 +813,6 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
 	       skb_shinfo(skb),
 	       offsetof(struct skb_shared_info, frags[skb_shinfo(skb)->nr_frags]));
 
-	/* Check if we can avoid taking references on fragments if we own
-	 * the last reference on skb->head. (see skb_release_data())
-	 */
-	if (!skb->cloned)
-		fastpath = true;
-	else {
-		int delta = skb->nohdr ? (1 << SKB_DATAREF_SHIFT) + 1 : 1;
-
-		fastpath = atomic_read(&skb_shinfo(skb)->dataref) == delta;
-	}
-
 	if (fastpath) {
 		kfree(skb->head);
 	} else {
@@ -816,6 +827,7 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
 	off = (data + nhead) - skb->head;
 
 	skb->head     = data;
+adjust_others:
 	skb->data    += off;
 #ifdef NET_SKBUFF_DATA_USES_OFFSET
 	skb->end      = size;
--
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