Subject: issue with inflight pages from page_pool

From: Lorenzo Bianconi <[email-masked]>
Date: Mon, 17 Apr 2023 19:53:17 +0200

Hi all,

I am triggering an issue with a device running the page_pool allocator.
In particular, the device is running an iperf tcp server receiving traffic
from a remote client. On the driver I loaded a simple xdp program returning
xdp_pass. When I remove the ebpf program and destroy the pool, page_pool
allocator starts complaining in page_pool_release_retry() that not all the pages
have been returned to the allocator. In fact, the pool is not really destroyed
in this case.
Debugging the code it seems the pages are stuck softnet_data defer_list and
they are never freed in skb_defer_free_flush() since I do not have any more tcp
traffic. To prove it, I tried to set sysctl_skb_defer_max to 0 and the issue
does not occur.
I developed the poc patch below and the issue seems to be fixed:

diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index 193c18799865..160f45c4e3a5 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -19,6 +19,7 @@
 #include <linux/mm.h> /* for put_page() */
 #include <linux/poison.h>
 #include <linux/ethtool.h>
+#include <linux/netdevice.h>
 
 #include <trace/events/page_pool.h>
 
@@ -810,12 +811,23 @@ static void page_pool_release_retry(struct work_struct *wq)
 {
 	struct delayed_work *dwq = to_delayed_work(wq);
 	struct page_pool *pool = container_of(dwq, typeof(*pool), release_dw);
-	int inflight;
+	int cpu, inflight;
 
 	inflight = page_pool_release(pool);
 	if (!inflight)
 		return;
 
+	/* Run NET_RX_SOFTIRQ in order to free pending skbs in softnet_data
+	 * defer_list that can stay in the list until we have enough queued
+	 * traffic.
+	 */
+	for_each_online_cpu(cpu) {
+		struct softnet_data *sd = &per_cpu(softnet_data, cpu);
+
+		if (!cmpxchg(&sd->defer_ipi_scheduled, 0, 1))
+			smp_call_function_single_async(cpu, &sd->defer_csd);
+	}
+
 	/* Periodic warning */
 	if (time_after_eq(jiffies, pool->defer_warn)) {
 		int sec = (s32)((u32)jiffies - (u32)pool->defer_start) / HZ;

Is it ok or do you think there is a better solution for issue?
@Felix: I think we faced a similar issue in mt76 unloading the module, right?

Regards,
Lorenzo