The patch titled
Subject: mm, frontswap: convert frontswap_enabled to static key
has been added to the -mm tree. Its filename is
mm-frontswap-convert-frontswap_enabled-to-static-key.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-frontswap-convert-frontswap_enabled-to-static-key.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-frontswap-convert-frontswap_enabled-to-static-key.patch
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From: Vlastimil Babka <vbabka@xxxxxxx>
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.
Link: http://lkml.kernel.org/r/1463152235-9717-1-git-send-email-vbabka@xxxxxxx
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
drivers/xen/xen-selfballoon.c | 4 +--
include/linux/frontswap.h | 34 ++++++++++++++++++-------------
mm/frontswap.c | 35 +++++++++++++-------------------
mm/swapfile.c | 2 -
4 files changed, 38 insertions(+), 37 deletions(-)
diff -puN drivers/xen/xen-selfballoon.c~mm-frontswap-convert-frontswap_enabled-to-static-key drivers/xen/xen-selfballoon.c
--- a/drivers/xen/xen-selfballoon.c~mm-frontswap-convert-frontswap_enabled-to-static-key
+++ a/drivers/xen/xen-selfballoon.c
@@ -195,7 +195,7 @@ static void selfballoon_process(struct w
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 w
reset_timer = true;
}
#ifdef CONFIG_FRONTSWAP
- if (frontswap_selfshrinking && frontswap_enabled) {
+ if (frontswap_selfshrinking) {
frontswap_selfshrink();
reset_timer = true;
}
diff -puN include/linux/frontswap.h~mm-frontswap-convert-frontswap_enabled-to-static-key include/linux/frontswap.h
--- a/include/linux/frontswap.h~mm-frontswap-convert-frontswap_enabled-to-static-key
+++ a/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(
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_m
#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_m
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);
}
diff -puN mm/frontswap.c~mm-frontswap-convert-frontswap_enabled-to-static-key mm/frontswap.c
--- a/mm/frontswap.c~mm-frontswap-convert-frontswap_enabled-to-static-key
+++ a/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 front
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, uns
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(unsigne
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(unsigne
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;
diff -puN mm/swapfile.c~mm-frontswap-convert-frontswap_enabled-to-static-key mm/swapfile.c
--- a/mm/swapfile.c~mm-frontswap-convert-frontswap_enabled-to-static-key
+++ a/mm/swapfile.c
@@ -2493,7 +2493,7 @@ SYSCALL_DEFINE2(swapon, const char __use
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)) {
_
Patches currently in -mm which might be from vbabka@xxxxxxx are
mm-page_alloc-prevent-infinite-loop-in-buffered_rmqueue.patch
mm-frontswap-convert-frontswap_enabled-to-static-key.patch
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