mm, frontswap: convert frontswap_enabled to static key

I have noticed that frontswap.h first declares "frontswap_enabled" as
extern bool variable, and then overrides it with "#define
frontswap_enabled (1)" for CONFIG_FRONTSWAP=Y or (0) when disabled.  The
bool variable isn't actually instantiated anywhere.

This all looks like an unfinished attempt to make frontswap_enabled
reflect whether a backend is instantiated.  But in the current state,
all frontswap hooks call unconditionally into frontswap.c just to check
if frontswap_ops is non-NULL.  This should at least be checked inline,
but we can further eliminate the overhead when CONFIG_FRONTSWAP is
enabled and no backend registered, using a static key that is initially
disabled, and gets enabled only upon first backend registration.

Thus, checks for "frontswap_enabled" are replaced with
"frontswap_enabled()" wrapping the static key check.  There are two
exceptions:

- xen's selfballoon_process() was testing frontswap_enabled in code guarded
  by #ifdef CONFIG_FRONTSWAP, which was effectively always true when reachable.
  The patch just removes this check. Using frontswap_enabled() does not sound
  correct here, as this can be true even without xen's own backend being
  registered.

- in SYSCALL_DEFINE2(swapon), change the check to IS_ENABLED(CONFIG_FRONTSWAP)
  as it seems the bitmap allocation cannot currently be postponed until a
  backend is registered. This means that frontswap will still have some
  memory overhead by being configured, but without a backend.

After the patch, we can expect that some functions in frontswap.c are
called only when frontswap_ops is non-NULL.  Change the checks there to
VM_BUG_ONs.  While at it, convert other BUG_ONs to VM_BUG_ONs as
frontswap has been stable for some time.

[akpm@linux-foundation.org: coding-style fixes]
Link: http://lkml.kernel.org/r/1463152235-9717-1-git-send-email-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/xen/xen-selfballoon.c

@@ -195,7 +195,7 @@ static void selfballoon_process(struct work_struct *work)
 				MB2PAGES(selfballoon_reserved_mb);
 #ifdef CONFIG_FRONTSWAP
 		/* allow space for frontswap pages to be repatriated */
-		if (frontswap_selfshrinking && frontswap_enabled)
+		if (frontswap_selfshrinking)
 			goal_pages += frontswap_curr_pages();
 #endif
 		if (cur_pages > goal_pages)
@@ -230,7 +230,7 @@ static void selfballoon_process(struct work_struct *work)
 		reset_timer = true;
 	}
 #ifdef CONFIG_FRONTSWAP
-	if (frontswap_selfshrinking && frontswap_enabled) {
+	if (frontswap_selfshrinking) {
 		frontswap_selfshrink();
 		reset_timer = true;
 	}

include/linux/frontswap.h

@@ -4,6 +4,7 @@
 #include <linux/swap.h>
 #include <linux/mm.h>
 #include <linux/bitops.h>
+#include <linux/jump_label.h>
 
 struct frontswap_ops {
 	void (*init)(unsigned); /* this swap type was just swapon'ed */
@@ -14,7 +15,6 @@ struct frontswap_ops {
 	struct frontswap_ops *next; /* private pointer to next ops */
 };
 
-extern bool frontswap_enabled;
 extern void frontswap_register_ops(struct frontswap_ops *ops);
 extern void frontswap_shrink(unsigned long);
 extern unsigned long frontswap_curr_pages(void);
@@ -30,7 +30,12 @@ extern void __frontswap_invalidate_page(unsigned, pgoff_t);
 extern void __frontswap_invalidate_area(unsigned);
 
 #ifdef CONFIG_FRONTSWAP
-#define frontswap_enabled (1)
+extern struct static_key_false frontswap_enabled_key;
+
+static inline bool frontswap_enabled(void)
+{
+	return static_branch_unlikely(&frontswap_enabled_key);
+}
 
 static inline bool frontswap_test(struct swap_info_struct *sis, pgoff_t offset)
 {
@@ -50,7 +55,10 @@ static inline unsigned long *frontswap_map_get(struct swap_info_struct *p)
 #else
 /* all inline routines become no-ops and all externs are ignored */
 
-#define frontswap_enabled (0)
+static inline bool frontswap_enabled(void)
+{
+	return false;
+}
 
 static inline bool frontswap_test(struct swap_info_struct *sis, pgoff_t offset)
 {
@@ -70,37 +78,35 @@ static inline unsigned long *frontswap_map_get(struct swap_info_struct *p)
 
 static inline int frontswap_store(struct page *page)
 {
-	int ret = -1;
+	if (frontswap_enabled())
+		return __frontswap_store(page);
 
-	if (frontswap_enabled)
-		ret = __frontswap_store(page);
-	return ret;
+	return -1;
 }
 
 static inline int frontswap_load(struct page *page)
 {
-	int ret = -1;
+	if (frontswap_enabled())
+		return __frontswap_load(page);
 
-	if (frontswap_enabled)
-		ret = __frontswap_load(page);
-	return ret;
+	return -1;
 }
 
 static inline void frontswap_invalidate_page(unsigned type, pgoff_t offset)
 {
-	if (frontswap_enabled)
+	if (frontswap_enabled())
 		__frontswap_invalidate_page(type, offset);
 }
 
 static inline void frontswap_invalidate_area(unsigned type)
 {
-	if (frontswap_enabled)
+	if (frontswap_enabled())
 		__frontswap_invalidate_area(type);
 }
 
 static inline void frontswap_init(unsigned type, unsigned long *map)
 {
-	if (frontswap_enabled)
+	if (frontswap_enabled())
 		__frontswap_init(type, map);
 }
 

mm/frontswap.c

@@ -20,6 +20,8 @@
 #include <linux/frontswap.h>
 #include <linux/swapfile.h>
 
+DEFINE_STATIC_KEY_FALSE(frontswap_enabled_key);
+
 /*
  * frontswap_ops are added by frontswap_register_ops, and provide the
  * frontswap "backend" implementation functions.  Multiple implementations
@@ -139,6 +141,8 @@ void frontswap_register_ops(struct frontswap_ops *ops)
 		ops->next = frontswap_ops;
 	} while (cmpxchg(&frontswap_ops, ops->next, ops) != ops->next);
 
+	static_branch_inc(&frontswap_enabled_key);
+
 	spin_lock(&swap_lock);
 	plist_for_each_entry(si, &swap_active_head, list) {
 		if (si->frontswap_map)
@@ -189,7 +193,7 @@ void __frontswap_init(unsigned type, unsigned long *map)
 	struct swap_info_struct *sis = swap_info[type];
 	struct frontswap_ops *ops;
 
-	BUG_ON(sis == NULL);
+	VM_BUG_ON(sis == NULL);
 
 	/*
 	 * p->frontswap is a bitmap that we MUST have to figure out which page
@@ -248,15 +252,9 @@ int __frontswap_store(struct page *page)
 	pgoff_t offset = swp_offset(entry);
 	struct frontswap_ops *ops;
 
-	/*
-	 * Return if no backend registed.
-	 * Don't need to inc frontswap_failed_stores here.
-	 */
-	if (!frontswap_ops)
-		return -1;
-
-	BUG_ON(!PageLocked(page));
-	BUG_ON(sis == NULL);
+	VM_BUG_ON(!frontswap_ops);
+	VM_BUG_ON(!PageLocked(page));
+	VM_BUG_ON(sis == NULL);
 
 	/*
 	 * If a dup, we must remove the old page first; we can't leave the
@@ -303,11 +301,10 @@ int __frontswap_load(struct page *page)
 	pgoff_t offset = swp_offset(entry);
 	struct frontswap_ops *ops;
 
-	if (!frontswap_ops)
-		return -1;
+	VM_BUG_ON(!frontswap_ops);
+	VM_BUG_ON(!PageLocked(page));
+	VM_BUG_ON(sis == NULL);
 
-	BUG_ON(!PageLocked(page));
-	BUG_ON(sis == NULL);
 	if (!__frontswap_test(sis, offset))
 		return -1;
 
@@ -337,10 +334,9 @@ void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
 	struct swap_info_struct *sis = swap_info[type];
 	struct frontswap_ops *ops;
 
-	if (!frontswap_ops)
-		return;
+	VM_BUG_ON(!frontswap_ops);
+	VM_BUG_ON(sis == NULL);
 
-	BUG_ON(sis == NULL);
 	if (!__frontswap_test(sis, offset))
 		return;
 
@@ -360,10 +356,9 @@ void __frontswap_invalidate_area(unsigned type)
 	struct swap_info_struct *sis = swap_info[type];
 	struct frontswap_ops *ops;
 
-	if (!frontswap_ops)
-		return;
+	VM_BUG_ON(!frontswap_ops);
+	VM_BUG_ON(sis == NULL);
 
-	BUG_ON(sis == NULL);
 	if (sis->frontswap_map == NULL)
 		return;
 

mm/swapfile.c

@@ -2493,7 +2493,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		goto bad_swap;
 	}
 	/* frontswap enabled? set up bit-per-page map for frontswap */
-	if (frontswap_enabled)
+	if (IS_ENABLED(CONFIG_FRONTSWAP))
 		frontswap_map = vzalloc(BITS_TO_LONGS(maxpages) * sizeof(long));
 
 	if (p->bdev &&(swap_flags & SWAP_FLAG_DISCARD) && swap_discardable(p)) {