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>

1 files changed, 18 insertions, 20 deletions

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));
         }