[PATCH NET v2 1/7] skbuff: introduce skb_expand_head()

[Vasily Averin <vvs@virtuozzo.com>]

Like skb_realloc_headroom(), new helper increases headroom of specified skb.
Unlike skb_realloc_headroom(), it does not allocate a new skb if possible;
copies skb->sk on new skb when as needed and frees original skb in case
of failures.

This helps to simplify ip[6]_finish_output2() and a few other similar cases.

Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
---
 include/linux/skbuff.h |  1 +
 net/core/skbuff.c      | 42 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index dbf820a..0003307 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1174,6 +1174,7 @@ static inline struct sk_buff *__pskb_copy(struct sk_buff *skb, int headroom,
 int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail, gfp_t gfp_mask);
 struct sk_buff *skb_realloc_headroom(struct sk_buff *skb,
 				     unsigned int headroom);
+struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom);
 struct sk_buff *skb_copy_expand(const struct sk_buff *skb, int newheadroom,
 				int newtailroom, gfp_t priority);
 int __must_check skb_to_sgvec_nomark(struct sk_buff *skb, struct scatterlist *sg,
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index bbc3b4b..a7997c2 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1769,6 +1769,48 @@ struct sk_buff *skb_realloc_headroom(struct sk_buff *skb, unsigned int headroom)
 EXPORT_SYMBOL(skb_realloc_headroom);
 
 /**
+ *	skb_expand_head - reallocate header of &sk_buff
+ *	@skb: buffer to reallocate
+ *	@headroom: needed headroom
+ *
+ *	Unlike skb_realloc_headroom, this one does not allocate a new skb
+ *	if possible; copies skb->sk to new skb as needed
+ *	and frees original skb in case of failures.
+ *
+ *	It expect increased headroom and generates warning otherwise.
+ */
+
+struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom)
+{
+	int delta = headroom - skb_headroom(skb);
+
+	if (WARN_ONCE(delta <= 0,
+		      "%s is expecting an increase in the headroom", __func__))
+		return skb;
+
+	/* pskb_expand_head() might crash, if skb is shared */
+	if (skb_shared(skb)) {
+		struct sk_buff *nskb = skb_clone(skb, GFP_ATOMIC);
+
+		if (likely(nskb)) {
+			if (skb->sk)
+				skb_set_owner_w(nskb, skb->sk);
+			consume_skb(skb);
+		} else {
+			kfree_skb(skb);
+		}
+		skb = nskb;
+	}
+	if (skb &&
+	    pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) {
+		kfree_skb(skb);
+		skb = NULL;
+	}
+	return skb;
+}
+EXPORT_SYMBOL(skb_expand_head);
+
+/**
  *	skb_copy_expand	-	copy and expand sk_buff
  *	@skb: buffer to copy
  *	@newheadroom: new free bytes at head
-- 
1.8.3.1