net: core: another layer of lists, around PF_MEMALLOC skb handling

First example of a layer splitting the list (rather than merely taking
 individual packets off it).
Involves new list.h function, list_cut_before(), like list_cut_position()
 but cuts on the other side of the given entry.
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/list.h

@@ -285,6 +285,36 @@ static inline void list_cut_position(struct list_head *list,
 		__list_cut_position(list, head, entry);
 }
 
+/**
+ * list_cut_before - cut a list into two, before given entry
+ * @list: a new list to add all removed entries
+ * @head: a list with entries
+ * @entry: an entry within head, could be the head itself
+ *
+ * This helper moves the initial part of @head, up to but
+ * excluding @entry, from @head to @list.  You should pass
+ * in @entry an element you know is on @head.  @list should
+ * be an empty list or a list you do not care about losing
+ * its data.
+ * If @entry == @head, all entries on @head are moved to
+ * @list.
+ */
+static inline void list_cut_before(struct list_head *list,
+				   struct list_head *head,
+				   struct list_head *entry)
+{
+	if (head->next == entry) {
+		INIT_LIST_HEAD(list);
+		return;
+	}
+	list->next = head->next;
+	list->next->prev = list;
+	list->prev = entry->prev;
+	list->prev->next = list;
+	head->next = entry;
+	entry->prev = head;
+}
+
 static inline void __list_splice(const struct list_head *list,
 				 struct list_head *prev,
 				 struct list_head *next)

net/core/dev.c

@@ -4784,6 +4784,14 @@ int netif_receive_skb_core(struct sk_buff *skb)
 }
 EXPORT_SYMBOL(netif_receive_skb_core);
 
+static void __netif_receive_skb_list_core(struct list_head *head, bool pfmemalloc)
+{
+	struct sk_buff *skb, *next;
+
+	list_for_each_entry_safe(skb, next, head, list)
+		__netif_receive_skb_core(skb, pfmemalloc);
+}
+
 static int __netif_receive_skb(struct sk_buff *skb)
 {
 	int ret;
@@ -4809,6 +4817,34 @@ static int __netif_receive_skb(struct sk_buff *skb)
 	return ret;
 }
 
+static void __netif_receive_skb_list(struct list_head *head)
+{
+	unsigned long noreclaim_flag = 0;
+	struct sk_buff *skb, *next;
+	bool pfmemalloc = false; /* Is current sublist PF_MEMALLOC? */
+
+	list_for_each_entry_safe(skb, next, head, list) {
+		if ((sk_memalloc_socks() && skb_pfmemalloc(skb)) != pfmemalloc) {
+			struct list_head sublist;
+
+			/* Handle the previous sublist */
+			list_cut_before(&sublist, head, &skb->list);
+			__netif_receive_skb_list_core(&sublist, pfmemalloc);
+			pfmemalloc = !pfmemalloc;
+			/* See comments in __netif_receive_skb */
+			if (pfmemalloc)
+				noreclaim_flag = memalloc_noreclaim_save();
+			else
+				memalloc_noreclaim_restore(noreclaim_flag);
+		}
+	}
+	/* Handle the remaining sublist */
+	__netif_receive_skb_list_core(head, pfmemalloc);
+	/* Restore pflags */
+	if (pfmemalloc)
+		memalloc_noreclaim_restore(noreclaim_flag);
+}
+
 static int generic_xdp_install(struct net_device *dev, struct netdev_bpf *xdp)
 {
 	struct bpf_prog *old = rtnl_dereference(dev->xdp_prog);
@@ -4843,14 +4879,6 @@ static int generic_xdp_install(struct net_device *dev, struct netdev_bpf *xdp)
 	return ret;
 }
 
-static void __netif_receive_skb_list(struct list_head *head)
-{
-	struct sk_buff *skb, *next;
-
-	list_for_each_entry_safe(skb, next, head, list)
-		__netif_receive_skb(skb);
-}
-
 static int netif_receive_skb_internal(struct sk_buff *skb)
 {
 	int ret;