[PATCH net v2 2/2] net: add a KUnit test for the alloc_skb_with_frags() tail split

[Haishuang Yan]

Allocate MAX_SKB_FRAGS - 2 full chunks of PAGE_ALLOC_COSTLY_ORDER plus
a 7 page tail. Rounding the tail down to 4 + 2 + 1 pages would need
one frag more than MAX_SKB_FRAGS, which made the allocation fail with
-ENOBUFS before "net: do not run out of frags in
alloc_skb_with_frags()".

If the allocation fails and no order-3 page can be had either, skip the
test instead of failing it, so that a host with fragmented memory does
not report this as a regression.

Signed-off-by: Haishuang Yan <yanhaishuang@xxxxxxxxxxxxxxxxxxxx>
---
 net/core/net_test.c | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/net/core/net_test.c b/net/core/net_test.c
index 2b5080c226636..de4d246f2f7ab 100644
--- a/net/core/net_test.c
+++ b/net/core/net_test.c
@@ -1721,7 +1721,42 @@ static void netmem_ref_test_tcp_devmem_recv(struct kunit *test)
 #endif
 #endif
 
+static void alloc_skb_with_frags_test_tail(struct kunit *test)
+{
+	/* MAX_SKB_FRAGS - 2 full chunks plus a 7 page tail: rounding the
+	 * tail down to 4 + 2 + 1 pages would need one frag too many.
+	 */
+	unsigned long len = (MAX_SKB_FRAGS - 2) *
+			    (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER) +
+			    7 * PAGE_SIZE;
+	unsigned long frags_len = 0;
+	struct sk_buff *skb;
+	int err, i;
+
+	skb = alloc_skb_with_frags(0, len, PAGE_ALLOC_COSTLY_ORDER, &err,
+				   GFP_KERNEL);
+	if (!skb) {
+		struct page *page;
+
+		/* Don't report a lack of high-order pages as a failure. */
+		page = alloc_pages(GFP_KERNEL | __GFP_NOWARN | __GFP_NORETRY,
+				   PAGE_ALLOC_COSTLY_ORDER);
+		if (!page)
+			kunit_skip(test, "no order-%d pages",
+				   PAGE_ALLOC_COSTLY_ORDER);
+		__free_pages(page, PAGE_ALLOC_COSTLY_ORDER);
+	}
+	KUNIT_ASSERT_NOT_NULL_MSG(test, skb, "err %d", err);
+
+	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
+		frags_len += skb_frag_size(&skb_shinfo(skb)->frags[i]);
+	KUNIT_EXPECT_EQ(test, frags_len, len);
+
+	kfree_skb(skb);
+}
+
 static struct kunit_case net_test_cases[] = {
+	KUNIT_CASE(alloc_skb_with_frags_test_tail),
 	KUNIT_CASE_PARAM(gso_test_func, gso_test_gen_params),
 	KUNIT_CASE_PARAM(ip_tunnel_flags_test_run,
 			 ip_tunnel_flags_test_gen_params),
-- 
2.43.0