[PATCH] neighbour: guarantee the localhost connections be established successfully even the ARP table is full

[Zheng Li <lizheng043@gmail.com>]

From: Zheng Li <James.Z.Li@Dell.com>

Inter-process communication on localhost should be established successfully even the ARP table is full,
many processes on server machine use the localhost to communicate such as command-line interface (CLI),
servers hope all CLI commands can be executed successfully even the arp table is full.
Right now CLI commands got timeout when the arp table is full.
Set the parameter of exempt_from_gc to be true for LOOPBACK net device to
keep localhost neigh in arp table, not removed by gc.

the steps of reproduced:
server with "gc_thresh3 = 1024" setting, ping server from more than 1024 IPv4 addresses,
run "ssh localhost" on console interface, then the command will get timeout.

Signed-off-by: Zheng Li <James.Z.Li@Dell.com>
---
 net/core/neighbour.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 552719c3bbc3..d96dee3d4af6 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -734,7 +734,10 @@ ___neigh_create(struct neigh_table *tbl, const void *pkey,
 struct neighbour *__neigh_create(struct neigh_table *tbl, const void *pkey,
 				 struct net_device *dev, bool want_ref)
 {
-	return ___neigh_create(tbl, pkey, dev, 0, false, want_ref);
+	if (dev->flags & IFF_LOOPBACK)
+		return ___neigh_create(tbl, pkey, dev, 0, true, want_ref);
+	else
+		return ___neigh_create(tbl, pkey, dev, 0, false, want_ref);
 }
 EXPORT_SYMBOL(__neigh_create);
 
-- 
2.17.1