SUNRPC: Fix a performance regression in the RPC authentication code

Fix a regression reported by Max Kellermann whereby kernel profiling
showed that his clients were spending 45% of their time in
rpcauth_lookup_credcache.

It turns out that although his processes had identical uid/gid/groups,
generic_match() was failing to detect this, because the task->group_info
pointers were not shared. This again lead to the creation of a huge number
of identical credentials at the RPC layer.

The regression is fixed by comparing the contents of task->group_info
if the actual pointers are not identical.
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

net/sunrpc/auth_generic.c

@@ -133,13 +133,29 @@ static int
 generic_match(struct auth_cred *acred, struct rpc_cred *cred, int flags)
 {
 	struct generic_cred *gcred = container_of(cred, struct generic_cred, gc_base);
+	int i;
 
 	if (gcred->acred.uid != acred->uid ||
 	    gcred->acred.gid != acred->gid ||
-	    gcred->acred.group_info != acred->group_info ||
 	    gcred->acred.machine_cred != acred->machine_cred)
-		return 0;
+		goto out_nomatch;
+
+	/* Optimisation in the case where pointers are identical... */
+	if (gcred->acred.group_info == acred->group_info)
+		goto out_match;
+
+	/* Slow path... */
+	if (gcred->acred.group_info->ngroups != acred->group_info->ngroups)
+		goto out_nomatch;
+	for (i = 0; i < gcred->acred.group_info->ngroups; i++) {
+		if (GROUP_AT(gcred->acred.group_info, i) !=
+				GROUP_AT(acred->group_info, i))
+			goto out_nomatch;
+	}
+out_match:
 	return 1;
+out_nomatch:
+	return 0;
 }
 
 void __init rpc_init_generic_auth(void)