frontswap: make frontswap_init use a pointer for the ops

This simplifies the code in the frontswap - we can get rid of the
'backend_registered' test and instead check against frontswap_ops.

[v1: Rebase on top of 703ba7fe5e0 (ramster->zcache move]
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>

4 files changed, 26 insertions, 28 deletions

diff --git a/drivers/staging/zcache/zcache-main.c b/drivers/staging/zcache/zcache-main.c
index e23d814b5392..09c69c8026f9 100644
--- a/drivers/staging/zcache/zcache-main.c
+++ b/drivers/staging/zcache/zcache-main.c
@@ -1707,9 +1707,9 @@ static struct frontswap_ops zcache_frontswap_ops = {
         .init = zcache_frontswap_init
 };
 
-struct frontswap_ops zcache_frontswap_register_ops(void)
+struct frontswap_ops *zcache_frontswap_register_ops(void)
 {
-        struct frontswap_ops old_ops =
+        struct frontswap_ops *old_ops =
                 frontswap_register_ops(&zcache_frontswap_ops);
 
         return old_ops;
@@ -1874,7 +1874,7 @@ static int __init zcache_init(void)
                         pr_warn("%s: cleancache_ops overridden\n", namestr);
         }
         if (zcache_enabled && !disable_frontswap) {
-                struct frontswap_ops old_ops;
+                struct frontswap_ops *old_ops;
 
                 old_ops = zcache_frontswap_register_ops();
                 if (frontswap_has_exclusive_gets)
@@ -1886,7 +1886,7 @@ static int __init zcache_init(void)
                         namestr, frontswap_has_exclusive_gets,
                         !disable_frontswap_ignore_nonactive);
 #endif
-                if (old_ops.init != NULL)
+                if (old_ops != NULL)
                         pr_warn("%s: frontswap_ops overridden\n", namestr);
         }
         if (ramster_enabled)
diff --git a/drivers/xen/tmem.c b/drivers/xen/tmem.c
index 3ee836d42581..7a01a5fd0f63 100644
--- a/drivers/xen/tmem.c
+++ b/drivers/xen/tmem.c
@@ -362,7 +362,7 @@ static int __init no_frontswap(char *s)
 }
 __setup("nofrontswap", no_frontswap);
 
-static struct frontswap_ops __initdata tmem_frontswap_ops = {
+static struct frontswap_ops tmem_frontswap_ops = {
         .store = tmem_frontswap_store,
         .load = tmem_frontswap_load,
         .invalidate_page = tmem_frontswap_flush_page,
@@ -378,11 +378,11 @@ static int __init xen_tmem_init(void)
 #ifdef CONFIG_FRONTSWAP
         if (tmem_enabled && use_frontswap) {
                 char *s = "";
-                struct frontswap_ops old_ops =
+                struct frontswap_ops *old_ops =
                         frontswap_register_ops(&tmem_frontswap_ops);
 
                 tmem_frontswap_poolid = -1;
-                if (old_ops.init != NULL)
+                if (old_ops)
                         s = " (WARNING: frontswap_ops overridden)";
                 printk(KERN_INFO "frontswap enabled, RAM provided by "
                                  "Xen Transcendent Memory%s\n", s);
diff --git a/include/linux/frontswap.h b/include/linux/frontswap.h
index 30442547b9e6..d4f29875c7cc 100644
--- a/include/linux/frontswap.h
+++ b/include/linux/frontswap.h
@@ -14,7 +14,7 @@ struct frontswap_ops {
 };
 
 extern bool frontswap_enabled;
-extern struct frontswap_ops
+extern struct frontswap_ops *
         frontswap_register_ops(struct frontswap_ops *ops);
 extern void frontswap_shrink(unsigned long);
 extern unsigned long frontswap_curr_pages(void);
diff --git a/mm/frontswap.c b/mm/frontswap.c
index cbd2b8af8129..e44c9cbd1443 100644
--- a/mm/frontswap.c
+++ b/mm/frontswap.c
@@ -24,7 +24,7 @@
  * frontswap_ops is set by frontswap_register_ops to contain the pointers
  * to the frontswap "backend" implementation functions.
  */
-static struct frontswap_ops frontswap_ops __read_mostly;
+static struct frontswap_ops *frontswap_ops __read_mostly;
 
 /*
  * This global enablement flag reduces overhead on systems where frontswap_ops
@@ -108,41 +108,39 @@ static inline void inc_frontswap_invalidates(void) { }
  *
  * The time between the backend being registered and the swap file system
  * calling the backend (via the frontswap_* functions) is indeterminate as
- * backend_registered is not atomic_t (or a value guarded by a spinlock).
+ * frontswap_ops is not atomic_t (or a value guarded by a spinlock).
  * That is OK as we are comfortable missing some of these calls to the newly
  * registered backend.
  *
  * Obviously the opposite (unloading the backend) must be done after all
  * the frontswap_[store|load|invalidate_area|invalidate_page] start
- * ignorning or failing the requests - at which point backend_registered
+ * ignorning or failing the requests - at which point frontswap_ops
  * would have to be made in some fashion atomic.
  */
 static DECLARE_BITMAP(need_init, MAX_SWAPFILES);
-static bool backend_registered __read_mostly;
 
 /*
  * Register operations for frontswap, returning previous thus allowing
  * detection of multiple backends and possible nesting.
  */
-struct frontswap_ops frontswap_register_ops(struct frontswap_ops *ops)
+struct frontswap_ops *frontswap_register_ops(struct frontswap_ops *ops)
 {
-        struct frontswap_ops old = frontswap_ops;
+        struct frontswap_ops *old = frontswap_ops;
         int i;
 
-        frontswap_ops = *ops;
         frontswap_enabled = true;
 
         for (i = 0; i < MAX_SWAPFILES; i++) {
                 if (test_and_clear_bit(i, need_init))
-                        (*frontswap_ops.init)(i);
+                        ops->init(i);
         }
         /*
-         * We MUST have backend_registered set _after_ the frontswap_init's
+         * We MUST have frontswap_ops set _after_ the frontswap_init's
          * have been called. Otherwise __frontswap_store might fail. Hence
          * the barrier to make sure compiler does not re-order us.
          */
         barrier();
-        backend_registered = true;
+        frontswap_ops = ops;
         return old;
 }
 EXPORT_SYMBOL(frontswap_register_ops);
@@ -172,11 +170,11 @@ void __frontswap_init(unsigned type)
 {
         struct swap_info_struct *sis = swap_info[type];
 
-        if (backend_registered) {
+        if (frontswap_ops) {
                 BUG_ON(sis == NULL);
                 if (sis->frontswap_map == NULL)
                         return;
-                (*frontswap_ops.init)(type);
+                frontswap_ops->init(type);
         } else {
                 BUG_ON(type > MAX_SWAPFILES);
                 set_bit(type, need_init);
@@ -206,7 +204,7 @@ int __frontswap_store(struct page *page)
         struct swap_info_struct *sis = swap_info[type];
         pgoff_t offset = swp_offset(entry);
 
-        if (!backend_registered) {
+        if (!frontswap_ops) {
                 inc_frontswap_failed_stores();
                 return ret;
         }
@@ -215,7 +213,7 @@ int __frontswap_store(struct page *page)
         BUG_ON(sis == NULL);
         if (frontswap_test(sis, offset))
                 dup = 1;
-        ret = frontswap_ops.store(type, offset, page);
+        ret = frontswap_ops->store(type, offset, page);
         if (ret == 0) {
                 frontswap_set(sis, offset);
                 inc_frontswap_succ_stores();
@@ -250,13 +248,13 @@ int __frontswap_load(struct page *page)
         struct swap_info_struct *sis = swap_info[type];
         pgoff_t offset = swp_offset(entry);
 
-        if (!backend_registered)
+        if (!frontswap_ops)
                 return ret;
 
         BUG_ON(!PageLocked(page));
         BUG_ON(sis == NULL);
         if (frontswap_test(sis, offset))
-                ret = frontswap_ops.load(type, offset, page);
+                ret = frontswap_ops->load(type, offset, page);
         if (ret == 0) {
                 inc_frontswap_loads();
                 if (frontswap_tmem_exclusive_gets_enabled) {
@@ -276,12 +274,12 @@ void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
 {
         struct swap_info_struct *sis = swap_info[type];
 
-        if (!backend_registered)
+        if (!frontswap_ops)
                 return;
 
         BUG_ON(sis == NULL);
         if (frontswap_test(sis, offset)) {
-                frontswap_ops.invalidate_page(type, offset);
+                frontswap_ops->invalidate_page(type, offset);
                 __frontswap_clear(sis, offset);
                 inc_frontswap_invalidates();
         }
@@ -296,11 +294,11 @@ void __frontswap_invalidate_area(unsigned type)
 {
         struct swap_info_struct *sis = swap_info[type];
 
-        if (backend_registered) {
+        if (frontswap_ops) {
                 BUG_ON(sis == NULL);
                 if (sis->frontswap_map == NULL)
                         return;
-                (*frontswap_ops.invalidate_area)(type);
+                frontswap_ops->invalidate_area(type);
                 atomic_set(&sis->frontswap_pages, 0);
                 memset(sis->frontswap_map, 0, sis->max / sizeof(long));
         }