net: Stop decapitating clones that have a head_frag

This change is meant ot prevent stealing the skb->head to use as a page in
the event that the skb->head was cloned.  This allows the other clones to
track each other via shinfo->dataref.

Without this we break down to two methods for tracking the reference count,
one being dataref, the other being the page count.  As a result it becomes
difficult to track how many references there are to skb->head.
Signed-off-by: NAlexander Duyck <alexander.h.duyck@intel.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Acked-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -1699,17 +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;
+	bool head_is_locked = !skb->head_frag || skb_cloned(skb);
 
 	/* 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 skb->head_frag is set, this 'linear' part is backed by a
+	 * fragment, and if the head is not shared with any clones then
+	 * we can avoid a copy since we own the head portion of this page.
 	 */
 	if (__splice_segment(virt_to_page(skb->data),
 			     (unsigned long) skb->data & (PAGE_SIZE - 1),
 			     skb_headlen(skb),
 			     offset, len, skb, spd,
-			     head_is_linear,
+			     head_is_locked,
 			     sk, pipe))
 		return true;
 

net/ipv4/tcp_input.c

@@ -4589,7 +4589,7 @@ static bool tcp_try_coalesce(struct sock *sk,
 		to->data_len += len;
 		goto merge;
 	}
-	if (from->head_frag) {
+	if (from->head_frag && !skb_cloned(from)) {
 		struct page *page;
 		unsigned int offset;
 
@@ -4599,12 +4599,7 @@ static bool tcp_try_coalesce(struct sock *sk,
 		offset = from->data - (unsigned char *)page_address(page);
 		skb_fill_page_desc(to, skb_shinfo(to)->nr_frags,
 				   page, offset, skb_headlen(from));
-
-		if (skb_cloned(from))
-			get_page(page);
-		else
-			*fragstolen = true;
-
+		*fragstolen = true;
 		delta = len; /* we dont know real truesize... */
 		goto copyfrags;
 	}