net: netpoll: flush skb pool during cleanup

The netpoll subsystem maintains a pool of 32 pre-allocated SKBs per
instance, but these SKBs are not freed when the netpoll user is brought
down. This leads to memory waste as these buffers remain allocated but
unused.

Add skb_pool_flush() to properly clean up these SKBs when netconsole is
terminated, improving memory efficiency.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20241114-skb_buffers_v2-v3-2-9be9f52a8b69@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: leitao

net/core/netpoll.c

@@ -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;
@@ -779,10 +787,12 @@ int netpoll_setup(struct netpoll *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)
@@ -830,6 +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);