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authorVlastimil Babka <vbabka@suse.cz>2016-07-26 15:24:42 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2016-07-26 16:19:19 -0700
commit8ea1d2a1985a7ae096edf5850a31d844ad1b8e97 (patch)
tree3ec2c4dc7d51fa1f5f2797e09fd8361a8404b474 /mm/frontswap.c
parentfbe84a09da746f781553051bb3dbc63f7b0a5162 (diff)
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>
Diffstat (limited to 'mm/frontswap.c')
-rw-r--r--mm/frontswap.c35
1 files changed, 15 insertions, 20 deletions
diff --git a/mm/frontswap.c b/mm/frontswap.c
index 27a9924caf61..fec8b5044040 100644
--- a/mm/frontswap.c
+++ b/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;