4 files changed, 18 insertions, 17 deletions

diff --git a/Documentation/vm/cleancache.txt b/Documentation/vm/cleancache.txt
index 01d76282444e..e4b49df7a048 100644
--- a/Documentation/vm/cleancache.txt
+++ b/Documentation/vm/cleancache.txt
@@ -28,9 +28,7 @@ IMPLEMENTATION OVERVIEW
 A cleancache "backend" that provides transcendent memory registers itself
 to the kernel's cleancache "frontend" by calling cleancache_register_ops,
 passing a pointer to a cleancache_ops structure with funcs set appropriately.
-Note that cleancache_register_ops returns the previous settings so that
-chaining can be performed if desired. The functions provided must conform to
-certain semantics as follows:
+The functions provided must conform to certain semantics as follows:
 
 Most important, cleancache is "ephemeral".  Pages which are copied into
 cleancache have an indefinite lifetime which is completely unknowable
diff --git a/drivers/xen/tmem.c b/drivers/xen/tmem.c
index 8a65423bc696..c4211a31612d 100644
--- a/drivers/xen/tmem.c
+++ b/drivers/xen/tmem.c
@@ -397,13 +397,15 @@ static int __init xen_tmem_init(void)
 #ifdef CONFIG_CLEANCACHE
         BUG_ON(sizeof(struct cleancache_filekey) != sizeof(struct tmem_oid));
         if (tmem_enabled && cleancache) {
-                char *s = "";
-                struct cleancache_ops *old_ops =
-                        cleancache_register_ops(&tmem_cleancache_ops);
-                if (old_ops)
-                        s = " (WARNING: cleancache_ops overridden)";
-                pr_info("cleancache enabled, RAM provided by Xen Transcendent Memory%s\n",
-                        s);
+                int err;
+
+                err = cleancache_register_ops(&tmem_cleancache_ops);
+                if (err)
+                        pr_warn("xen-tmem: failed to enable cleancache: %d\n",
+                                err);
+                else
+                        pr_info("cleancache enabled, RAM provided by "
+                                "Xen Transcendent Memory\n");
         }
 #endif
 #ifdef CONFIG_XEN_SELFBALLOONING
diff --git a/include/linux/cleancache.h b/include/linux/cleancache.h
index 29657d1c83fb..b23611f43cfb 100644
--- a/include/linux/cleancache.h
+++ b/include/linux/cleancache.h
@@ -33,8 +33,7 @@ struct cleancache_ops {
         void (*invalidate_fs)(int);
 };
 
-extern struct cleancache_ops *
-        cleancache_register_ops(struct cleancache_ops *ops);
+extern int cleancache_register_ops(struct cleancache_ops *ops);
 extern void __cleancache_init_fs(struct super_block *);
 extern void __cleancache_init_shared_fs(struct super_block *);
 extern int  __cleancache_get_page(struct page *);
diff --git a/mm/cleancache.c b/mm/cleancache.c
index 532495f2e4f4..aa10f9a3bc88 100644
--- a/mm/cleancache.c
+++ b/mm/cleancache.c
@@ -106,15 +106,17 @@ static DEFINE_MUTEX(poolid_mutex);
  */
 
 /*
- * Register operations for cleancache, returning previous thus allowing
- * detection of multiple backends and possible nesting.
+ * Register operations for cleancache. Returns 0 on success.
  */
-struct cleancache_ops *cleancache_register_ops(struct cleancache_ops *ops)
+int cleancache_register_ops(struct cleancache_ops *ops)
 {
-        struct cleancache_ops *old = cleancache_ops;
         int i;
 
         mutex_lock(&poolid_mutex);
+        if (cleancache_ops) {
+                mutex_unlock(&poolid_mutex);
+                return -EBUSY;
+        }
         for (i = 0; i < MAX_INITIALIZABLE_FS; i++) {
                 if (fs_poolid_map[i] == FS_NO_BACKEND)
                         fs_poolid_map[i] = ops->init_fs(PAGE_SIZE);
@@ -130,7 +132,7 @@ struct cleancache_ops *cleancache_register_ops(struct cleancache_ops *ops)
         barrier();
         cleancache_ops = ops;
         mutex_unlock(&poolid_mutex);
-        return old;
+        return 0;
 }
 EXPORT_SYMBOL(cleancache_register_ops);