zcache/tmem: Better error checking on frontswap_register_ops return value.

In the past it either used to be NULL or the "older" backend. Now we
also return -Exx error codes.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Bob Liu <lliubbo@gmail.com>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: Andor Daam <andor.daam@googlemail.com>
Cc: Dan Magenheimer <dan.magenheimer@oracle.com>
Cc: Florian Schmaus <fschmaus@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Stefan Hengelein <ilendir@googlemail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/staging/zcache/zcache-main.c

@@ -1886,8 +1886,11 @@ static int __init zcache_init(void)
 			namestr, frontswap_has_exclusive_gets,
 			!disable_frontswap_ignore_nonactive);
 #endif
-		if (old_ops != NULL)
+		if (IS_ERR(old_ops) || old_ops) {
+			if (IS_ERR(old_ops))
+				return PTR_RET(old_ops);
 			pr_warn("%s: frontswap_ops overridden\n", namestr);
+		}
 	}
 	if (ramster_enabled)
 		ramster_init(!disable_cleancache, !disable_frontswap,

drivers/xen/tmem.c

@@ -397,8 +397,11 @@ static int xen_tmem_init(void)
 			frontswap_register_ops(&tmem_frontswap_ops);
 
 		tmem_frontswap_poolid = -1;
-		if (old_ops)
+		if (IS_ERR(old_ops) || old_ops) {
+			if (IS_ERR(old_ops))
+				return PTR_ERR(old_ops);
 			s = " (WARNING: frontswap_ops overridden)";
+		}
 		printk(KERN_INFO "frontswap enabled, RAM provided by "
 				 "Xen Transcendent Memory%s\n", s);
 	}