net: add LINUX_MIB_PFMEMALLOCDROP counter

Debugging issues caused by pfmemalloc is often tedious.

Add a new SNMP counter to more easily diagnose these problems.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Josef Bacik <jbacik@fb.com>
Acked-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/snmp.h

@@ -240,6 +240,7 @@ enum
 	LINUX_MIB_SACKMERGED,
 	LINUX_MIB_SACKSHIFTFALLBACK,
 	LINUX_MIB_TCPBACKLOGDROP,
+	LINUX_MIB_PFMEMALLOCDROP,
 	LINUX_MIB_TCPMINTTLDROP, /* RFC 5082 */
 	LINUX_MIB_TCPDEFERACCEPTDROP,
 	LINUX_MIB_IPRPFILTER, /* IP Reverse Path Filter (rp_filter) */

net/core/filter.c

@@ -76,9 +76,10 @@ int sk_filter_trim_cap(struct sock *sk, struct sk_buff *skb, unsigned int cap)
 	 * allow SOCK_MEMALLOC sockets to use it as this socket is
 	 * helping free memory
 	 */
-	if (skb_pfmemalloc(skb) && !sock_flag(sk, SOCK_MEMALLOC))
+	if (skb_pfmemalloc(skb) && !sock_flag(sk, SOCK_MEMALLOC)) {
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_PFMEMALLOCDROP);
 		return -ENOMEM;
-
+	}
 	err = BPF_CGROUP_RUN_PROG_INET_INGRESS(sk, skb);
 	if (err)
 		return err;

net/ipv4/proc.c

@@ -262,6 +262,7 @@ static const struct snmp_mib snmp4_net_list[] = {
 	SNMP_MIB_ITEM("TCPSackMerged", LINUX_MIB_SACKMERGED),
 	SNMP_MIB_ITEM("TCPSackShiftFallback", LINUX_MIB_SACKSHIFTFALLBACK),
 	SNMP_MIB_ITEM("TCPBacklogDrop", LINUX_MIB_TCPBACKLOGDROP),
+	SNMP_MIB_ITEM("PFMemallocDrop", LINUX_MIB_PFMEMALLOCDROP),
 	SNMP_MIB_ITEM("TCPMinTTLDrop", LINUX_MIB_TCPMINTTLDROP),
 	SNMP_MIB_ITEM("TCPDeferAcceptDrop", LINUX_MIB_TCPDEFERACCEPTDROP),
 	SNMP_MIB_ITEM("IPReversePathFilter", LINUX_MIB_IPRPFILTER),