[PATCH] net: add big honking pfmemalloc OOM warning

[Juha-Matti Tilli]

A system administrator is not notified (except via an obscure SNMP
counter that most sysadmins don't know to look for) if packets are
dropped due to out-of-memory condition when SKBs use pfmemalloc
reserves. This can for example lead to NFS connections hanging on
high-volume systems.

Implement a ratelimited big honking out of memory warning that directs
the sysadmin to bump up vm.min_free_kbytes in case this problem happens.
Our experience shows that with default vm.min_free_kbytes (90112, or
about 90 megabytes), NFS connections hang approximately once per day,
whereas with 901120 (default multiplied by 10) NFS connections never
hang.

Signed-off-by: Juha-Matti Tilli <juha-matti.tilli@xxxxxxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Rafael Aquini <aquini@xxxxxxxxxx>
Cc: Murphy Zhou <xzhou@xxxxxxxxxx>
Cc: Yongcheng Yang <yoyang@xxxxxxxxxx>
Cc: Jianhong Yin <jiyin@xxxxxxxxxx>
---
 net/core/filter.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index fc92ebc4e200..7d8ef239b9af 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -99,6 +99,8 @@ int sk_filter_trim_cap(struct sock *sk, struct sk_buff *skb, unsigned int cap)
 	 * helping free memory
 	 */
 	if (skb_pfmemalloc(skb) && !sock_flag(sk, SOCK_MEMALLOC)) {
+		printk_ratelimited(KERN_WARNING
+				   "dropped packet due to out-of-memory condition, please bump up vm.min_free_kbytes\n");
 		NET_INC_STATS(sock_net(sk), LINUX_MIB_PFMEMALLOCDROP);
 		return -ENOMEM;
 	}
-- 
2.17.1