[v3 net-next 08/10] skbuff: reuse NAPI skb cache on allocation path (__build_skb())

[Alexander Lobakin <alobakin@pm.me>]

Instead of just bulk-flushing skbuff_heads queued up through
napi_consume_skb() or __kfree_skb_defer(), try to reuse them
on allocation path.
If the cache is empty on allocation, bulk-allocate the first
half, which is more efficient than per-skb allocation.
If the cache is full on freeing, bulk-wipe the second half.
This also includes custom KASAN poisoning/unpoisoning to be
double sure there are no use-after-free cases.

Functions that got cache fastpath:
 - {,__}build_skb();
 - {,__}netdev_alloc_skb();
 - {,__}napi_alloc_skb().

Note on "napi_safe" argument:
NAPI cache should be accessed only from BH-disabled or (better)
NAPI context. To make sure access is safe, in_serving_softirq()
check is used.
Hovewer, there are plenty of cases when we know for sure that
we're in such context. This includes: build_skb() (called only
from NIC drivers in NAPI Rx context) and {,__}napi_alloc_skb()
(called from the same place or from kernel network softirq
functions).
We can use that knowledge to avoid unnecessary checks.

Suggested-by: Edward Cree <ecree.xilinx@gmail.com> # Unified cache part
Suggested-by: Eric Dumazet <edumazet@google.com>   # KASAN poisoning
Suggested-by: Dmitry Vyukov <dvyukov@google.com>   # Help with KASAN
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
---
 include/linux/skbuff.h   |  2 +-
 net/core/skbuff.c        | 61 ++++++++++++++++++++++++++++------------
 net/netlink/af_netlink.c |  2 +-
 3 files changed, 45 insertions(+), 20 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 0e0707296098..5bb443d37bf4 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1082,7 +1082,7 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
 
 struct sk_buff *__alloc_skb(unsigned int size, gfp_t priority, int flags,
 			    int node);
-struct sk_buff *__build_skb(void *data, unsigned int frag_size);
+struct sk_buff *__build_skb(void *data, unsigned int frag_size, bool napi_safe);
 struct sk_buff *build_skb(void *data, unsigned int frag_size);
 struct sk_buff *build_skb_around(struct sk_buff *skb,
 				 void *data, unsigned int frag_size);
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 860a9d4f752f..8747566a8136 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -120,6 +120,7 @@ static void skb_under_panic(struct sk_buff *skb, unsigned int sz, void *addr)
 }
 
 #define NAPI_SKB_CACHE_SIZE	64
+#define NAPI_SKB_CACHE_HALF	(NAPI_SKB_CACHE_SIZE / 2)
 
 struct napi_alloc_cache {
 	struct page_frag_cache page;
@@ -164,6 +165,30 @@ void *__netdev_alloc_frag_align(unsigned int fragsz, unsigned int align_mask)
 }
 EXPORT_SYMBOL(__netdev_alloc_frag_align);
 
+static struct sk_buff *napi_skb_cache_get(bool napi_safe)
+{
+	struct napi_alloc_cache *nc;
+	struct sk_buff *skb;
+
+	if (!napi_safe && unlikely(!in_serving_softirq()))
+		return kmem_cache_alloc(skbuff_head_cache, GFP_ATOMIC);
+
+	nc = this_cpu_ptr(&napi_alloc_cache);
+
+	if (unlikely(!nc->skb_count))
+		nc->skb_count = kmem_cache_alloc_bulk(skbuff_head_cache,
+						      GFP_ATOMIC,
+						      NAPI_SKB_CACHE_HALF,
+						      nc->skb_cache);
+	if (unlikely(!nc->skb_count))
+		return NULL;
+
+	skb = nc->skb_cache[--nc->skb_count];
+	kasan_unpoison_object_data(skbuff_head_cache, skb);
+
+	return skb;
+}
+
 /* Caller must provide SKB that is memset cleared */
 static void __build_skb_around(struct sk_buff *skb, void *data,
 			       unsigned int frag_size)
@@ -210,11 +235,11 @@ static void __build_skb_around(struct sk_buff *skb, void *data,
  *  before giving packet to stack.
  *  RX rings only contains data buffers, not full skbs.
  */
-struct sk_buff *__build_skb(void *data, unsigned int frag_size)
+struct sk_buff *__build_skb(void *data, unsigned int frag_size, bool napi_safe)
 {
 	struct sk_buff *skb;
 
-	skb = kmem_cache_alloc(skbuff_head_cache, GFP_ATOMIC);
+	skb = napi_skb_cache_get(napi_safe);
 	if (unlikely(!skb))
 		return NULL;
 
@@ -231,7 +256,7 @@ struct sk_buff *__build_skb(void *data, unsigned int frag_size)
  */
 struct sk_buff *build_skb(void *data, unsigned int frag_size)
 {
-	struct sk_buff *skb = __build_skb(data, frag_size);
+	struct sk_buff *skb = __build_skb(data, frag_size, true);
 
 	if (skb && frag_size) {
 		skb->head_frag = 1;
@@ -443,7 +468,7 @@ struct sk_buff *__netdev_alloc_skb(struct net_device *dev, unsigned int len,
 	if (unlikely(!data))
 		return NULL;
 
-	skb = __build_skb(data, len);
+	skb = __build_skb(data, len, false);
 	if (unlikely(!skb)) {
 		skb_free_frag(data);
 		return NULL;
@@ -507,7 +532,7 @@ struct sk_buff *__napi_alloc_skb(struct napi_struct *napi, unsigned int len,
 	if (unlikely(!data))
 		return NULL;
 
-	skb = __build_skb(data, len);
+	skb = __build_skb(data, len, true);
 	if (unlikely(!skb)) {
 		skb_free_frag(data);
 		return NULL;
@@ -838,31 +863,31 @@ void __consume_stateless_skb(struct sk_buff *skb)
 	kfree_skbmem(skb);
 }
 
-static inline void _kfree_skb_defer(struct sk_buff *skb)
+static void napi_skb_cache_put(struct sk_buff *skb)
 {
 	struct napi_alloc_cache *nc = this_cpu_ptr(&napi_alloc_cache);
+	u32 i;
 
 	/* drop skb->head and call any destructors for packet */
 	skb_release_all(skb);
 
-	/* record skb to CPU local list */
+	kasan_poison_object_data(skbuff_head_cache, skb);
 	nc->skb_cache[nc->skb_count++] = skb;
 
-#ifdef CONFIG_SLUB
-	/* SLUB writes into objects when freeing */
-	prefetchw(skb);
-#endif
-
-	/* flush skb_cache if it is filled */
 	if (unlikely(nc->skb_count == NAPI_SKB_CACHE_SIZE)) {
-		kmem_cache_free_bulk(skbuff_head_cache, NAPI_SKB_CACHE_SIZE,
-				     nc->skb_cache);
-		nc->skb_count = 0;
+		for (i = NAPI_SKB_CACHE_HALF; i < NAPI_SKB_CACHE_SIZE; i++)
+			kasan_unpoison_object_data(skbuff_head_cache,
+						   nc->skb_cache[i]);
+
+		kmem_cache_free_bulk(skbuff_head_cache, NAPI_SKB_CACHE_HALF,
+				     nc->skb_cache + NAPI_SKB_CACHE_HALF);
+		nc->skb_count = NAPI_SKB_CACHE_HALF;
 	}
 }
+
 void __kfree_skb_defer(struct sk_buff *skb)
 {
-	_kfree_skb_defer(skb);
+	napi_skb_cache_put(skb);
 }
 
 void napi_consume_skb(struct sk_buff *skb, int budget)
@@ -887,7 +912,7 @@ void napi_consume_skb(struct sk_buff *skb, int budget)
 		return;
 	}
 
-	_kfree_skb_defer(skb);
+	napi_skb_cache_put(skb);
 }
 EXPORT_SYMBOL(napi_consume_skb);
 
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index dd488938447f..afba4e11a526 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -1190,7 +1190,7 @@ static struct sk_buff *netlink_alloc_large_skb(unsigned int size,
 	if (data == NULL)
 		return NULL;
 
-	skb = __build_skb(data, size);
+	skb = __build_skb(data, size, false);
 	if (skb == NULL)
 		vfree(data);
 	else
-- 
2.30.0