[PATCH net-next] net: add skbuff_clear() helper

[Eric Dumazet <edumazet@google.com>]

clang is unable to inline the memset() calls in net/core/skbuff.c
when initializing allocated sk_buff.

memset(skb, 0, offsetof(struct sk_buff, tail));

This is unfortunate, because:

1) calling external memset_orig() helper adds a call/ret and
   typical setup cost.

2) offsetof(struct sk_buff, tail) == 0xb8 = 0x80 + 0x38

   On x86_64, memset_orig() performs two 64 bytes clear,
   then has to loop 7 times to clear the final 56 bytes.

skbuff_clear() makes sure the minimal and optimal code
is generated.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/core/skbuff.c | 32 ++++++++++++++++++++++----------
 1 file changed, 22 insertions(+), 10 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index a56133902c0d9c47b45a4a19b228b151456e5051..4887099e8678352a62d805e1b0be2736dc985376 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -307,6 +307,23 @@ static struct sk_buff *napi_skb_cache_get(bool alloc)
 	return skb;
 }
 
+/*
+ * Only clear those fields we need to clear, not those that we will
+ * actually initialise later. Hence, don't put any more fields after
+ * the tail pointer in struct sk_buff!
+ */
+static inline void skbuff_clear(struct sk_buff *skb)
+{
+	/* Replace memset(skb, 0, offsetof(struct sk_buff, tail))
+	 * with two smaller memset(), with a barrier() between them.
+	 * This forces the compiler to inline both calls.
+	 */
+	BUILD_BUG_ON(offsetof(struct sk_buff, tail) <= 128);
+	memset(skb, 0, 128);
+	barrier();
+	memset((void *)skb + 128, 0, offsetof(struct sk_buff, tail) - 128);
+}
+
 /**
  * napi_skb_cache_get_bulk - obtain a number of zeroed skb heads from the cache
  * @skbs: pointer to an at least @n-sized array to fill with skb pointers
@@ -357,7 +374,7 @@ u32 napi_skb_cache_get_bulk(void **skbs, u32 n)
 		skbs[i] = nc->skb_cache[base + i];
 
 		kasan_mempool_unpoison_object(skbs[i], skbuff_cache_size);
-		memset(skbs[i], 0, offsetof(struct sk_buff, tail));
+		skbuff_clear(skbs[i]);
 	}
 
 	nc->skb_count -= n;
@@ -424,7 +441,7 @@ struct sk_buff *slab_build_skb(void *data)
 	if (unlikely(!skb))
 		return NULL;
 
-	memset(skb, 0, offsetof(struct sk_buff, tail));
+	skbuff_clear(skb);
 	data = __slab_build_skb(data, &size);
 	__finalize_skb_around(skb, data, size);
 
@@ -476,7 +493,7 @@ struct sk_buff *__build_skb(void *data, unsigned int frag_size)
 	if (unlikely(!skb))
 		return NULL;
 
-	memset(skb, 0, offsetof(struct sk_buff, tail));
+	skbuff_clear(skb);
 	__build_skb_around(skb, data, frag_size);
 
 	return skb;
@@ -537,7 +554,7 @@ static struct sk_buff *__napi_build_skb(void *data, unsigned int frag_size)
 	if (unlikely(!skb))
 		return NULL;
 
-	memset(skb, 0, offsetof(struct sk_buff, tail));
+	skbuff_clear(skb);
 	__build_skb_around(skb, data, frag_size);
 
 	return skb;
@@ -696,12 +713,7 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
 	 */
 	prefetchw(data + SKB_WITH_OVERHEAD(size));
 
-	/*
-	 * Only clear those fields we need to clear, not those that we will
-	 * actually initialise below. Hence, don't put any more fields after
-	 * the tail pointer in struct sk_buff!
-	 */
-	memset(skb, 0, offsetof(struct sk_buff, tail));
+	skbuff_clear(skb);
 	__build_skb_around(skb, data, size);
 	skb->pfmemalloc = pfmemalloc;
 
-- 
2.52.0.457.g6b5491de43-goog