From 8ea1d2a1985a7ae096edf5850a31d844ad1b8e97 Mon Sep 17 00:00:00 2001 From: Vlastimil Babka Date: Tue, 26 Jul 2016 15:24:42 -0700 Subject: 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 Cc: Konrad Rzeszutek Wilk Cc: Boris Ostrovsky Cc: David Vrabel Cc: Juergen Gross Cc: "Kirill A. Shutemov" Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/frontswap.h | 34 ++++++++++++++++++++-------------- 1 file changed, 20 insertions(+), 14 deletions(-) (limited to 'include/linux/frontswap.h') diff --git a/include/linux/frontswap.h b/include/linux/frontswap.h index e65ef959546c..c46d2aa16d81 100644 --- a/include/linux/frontswap.h +++ b/include/linux/frontswap.h @@ -4,6 +4,7 @@ #include #include #include +#include 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); } -- cgit v1.2.3