Merge branch 'net-netpoll-improve-skb-pool-management'

Breno Leitao says:

====================
net: netpoll: Improve SKB pool management

The netpoll subsystem pre-allocates 32 SKBs in a pool for emergency use
during out-of-memory conditions. However, the current implementation has
several inefficiencies:

 * The SKB pool, once allocated, is never freed:
	 * Resources remain allocated even after netpoll users are removed
	 * Failed initialization can leave pool populated forever
 * The global pool design makes resource tracking difficult

This series addresses these issues through three patches:

Patch 1 ("net: netpoll: Individualize the skb pool"):
 - Replace global pool with per-user pools in netpoll struct

Patch 2 ("net: netpoll: flush skb pool during cleanup"):
- Properly free pool resources during netconsole cleanup

These changes improve resource management and make the code more
maintainable.  As a side benefit, the improved structure would allow
netpoll to be modularized if desired in the future.

v2: https://lore.kernel.org/20241107-skb_buffers_v2-v2-0-288c6264ba4f@debian.org
v1: https://lore.kernel.org/20241025142025.3558051-1-leitao@debian.org
====================

Link: https://patch.msgid.link/20241114-skb_buffers_v2-v3-0-9be9f52a8b69@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kuba-moo

include/linux/netpoll.h

@@ -32,6 +32,7 @@ struct netpoll {
 	bool ipv6;
 	u16 local_port, remote_port;
 	u8 remote_mac[ETH_ALEN];
+	struct sk_buff_head skb_pool;
 };
 
 struct netpoll_info {

net/core/netpoll.c

@@ -45,9 +45,6 @@
 
 #define MAX_UDP_CHUNK 1460
 #define MAX_SKBS 32
-
-static struct sk_buff_head skb_pool;
-
 #define USEC_PER_POLL	50
 
 #define MAX_SKB_SIZE							\
@@ -234,20 +231,23 @@ void netpoll_poll_enable(struct net_device *dev)
 		up(&ni->dev_lock);
 }
 
-static void refill_skbs(void)
+static void refill_skbs(struct netpoll *np)
 {
+	struct sk_buff_head *skb_pool;
 	struct sk_buff *skb;
 	unsigned long flags;
 
-	spin_lock_irqsave(&skb_pool.lock, flags);
-	while (skb_pool.qlen < MAX_SKBS) {
+	skb_pool = &np->skb_pool;
+
+	spin_lock_irqsave(&skb_pool->lock, flags);
+	while (skb_pool->qlen < MAX_SKBS) {
 		skb = alloc_skb(MAX_SKB_SIZE, GFP_ATOMIC);
 		if (!skb)
 			break;
 
-		__skb_queue_tail(&skb_pool, skb);
+		__skb_queue_tail(skb_pool, skb);
 	}
-	spin_unlock_irqrestore(&skb_pool.lock, flags);
+	spin_unlock_irqrestore(&skb_pool->lock, flags);
 }
 
 static void zap_completion_queue(void)
@@ -284,12 +284,12 @@ static struct sk_buff *find_skb(struct netpoll *np, int len, int reserve)
 	struct sk_buff *skb;
 
 	zap_completion_queue();
-	refill_skbs();
+	refill_skbs(np);
 repeat:
 
 	skb = alloc_skb(len, GFP_ATOMIC);
 	if (!skb)
-		skb = skb_dequeue(&skb_pool);
+		skb = skb_dequeue(&np->skb_pool);
 
 	if (!skb) {
 		if (++count < 10) {
@@ -531,6 +531,14 @@ static int netpoll_parse_ip_addr(const char *str, union inet_addr *addr)
 	return -1;
 }
 
+static void skb_pool_flush(struct netpoll *np)
+{
+	struct sk_buff_head *skb_pool;
+
+	skb_pool = &np->skb_pool;
+	skb_queue_purge_reason(skb_pool, SKB_CONSUMED);
+}
+
 int netpoll_parse_options(struct netpoll *np, char *opt)
 {
 	char *cur=opt, *delim;
@@ -673,6 +681,8 @@ int netpoll_setup(struct netpoll *np)
 	struct in_device *in_dev;
 	int err;
 
+	skb_queue_head_init(&np->skb_pool);
+
 	rtnl_lock();
 	if (np->dev_name[0]) {
 		struct net *net = current->nsproxy->net_ns;
@@ -773,14 +783,16 @@ int netpoll_setup(struct netpoll *np)
 	}
 
 	/* fill up the skb queue */
-	refill_skbs();
+	refill_skbs(np);
 
 	err = __netpoll_setup(np, ndev);
 	if (err)
-		goto put;
+		goto flush;
 	rtnl_unlock();
 	return 0;
 
+flush:
+	skb_pool_flush(np);
 put:
 	DEBUG_NET_WARN_ON_ONCE(np->dev);
 	if (ip_overwritten)
@@ -792,13 +804,6 @@ int netpoll_setup(struct netpoll *np)
 }
 EXPORT_SYMBOL(netpoll_setup);
 
-static int __init netpoll_init(void)
-{
-	skb_queue_head_init(&skb_pool);
-	return 0;
-}
-core_initcall(netpoll_init);
-
 static void rcu_cleanup_netpoll_info(struct rcu_head *rcu_head)
 {
 	struct netpoll_info *npinfo =
@@ -835,6 +840,8 @@ void __netpoll_cleanup(struct netpoll *np)
 		call_rcu(&npinfo->rcu, rcu_cleanup_netpoll_info);
 	} else
 		RCU_INIT_POINTER(np->dev->npinfo, NULL);
+
+	skb_pool_flush(np);
 }
 EXPORT_SYMBOL_GPL(__netpoll_cleanup);