[PATCH v3 1/6] net: pad skb data and shinfo as a whole rather than individually

[Ian Campbell <ian.campbell@citrix.com>]

This reduces the minimum overhead required for this allocation such that the
shinfo can be grown in the following patch without overflowing 2048 bytes for a
1500 byte frame.

Reducing this overhead while also growing the shinfo means that sometimes the
tail end of the data can end up in the same cache line as the beginning of the
shinfo. Specifically in the case of the 64 byte cache lines on a 64 bit system
the first 8 bytes of shinfo can overlap the tail cacheline of the data. In many
cases the allocation slop means that there is no overlap.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
---
 include/linux/skbuff.h |    4 ++--
 net/core/skbuff.c      |    3 +--
 2 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 50db9b0..f04333b 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -41,7 +41,7 @@
 #define SKB_DATA_ALIGN(X)	(((X) + (SMP_CACHE_BYTES - 1)) & \
 				 ~(SMP_CACHE_BYTES - 1))
 #define SKB_WITH_OVERHEAD(X)	\
-	((X) - SKB_DATA_ALIGN(sizeof(struct skb_shared_info)))
+	((X) - sizeof(struct skb_shared_info))
 #define SKB_MAX_ORDER(X, ORDER) \
 	SKB_WITH_OVERHEAD((PAGE_SIZE << (ORDER)) - (X))
 #define SKB_MAX_HEAD(X)		(SKB_MAX_ORDER((X), 0))
@@ -50,7 +50,7 @@
 /* return minimum truesize of one skb containing X bytes of data */
 #define SKB_TRUESIZE(X) ((X) +						\
 			 SKB_DATA_ALIGN(sizeof(struct sk_buff)) +	\
-			 SKB_DATA_ALIGN(sizeof(struct skb_shared_info)))
+			 sizeof(struct skb_shared_info))
 
 /* A. Checksumming of received packets by device.
  *
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index da0c97f..b6de604 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -189,8 +189,7 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
 	 * aligned memory blocks, unless SLUB/SLAB debug is enabled.
 	 * Both skb->head and skb_shared_info are cache line aligned.
 	 */
-	size = SKB_DATA_ALIGN(size);
-	size += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
+	size = SKB_DATA_ALIGN(size + sizeof(struct skb_shared_info));
 	data = kmalloc_node_track_caller(size, gfp_mask, node);
 	if (!data)
 		goto nodata;
-- 
1.7.2.5