Split the vm_page flags PG_WRITEABLE and PG_REFERENCED into atomic

flags field. Updates to the atomic flags are performed using the atomic
ops on the containing word, do not require any vm lock to be held, and
are non-blocking. The vm_page_aflag_set(9) and vm_page_aflag_clear(9)
functions are provided to modify afalgs.

Document the changes to flags field to only require the page lock.

Introduce vm_page_reference(9) function to provide a stable KPI and
KBI for filesystems like tmpfs and zfs which need to mark a page as
referenced.

Reviewed by:    alc, attilio
Tested by:      marius, flo (sparc64); andreast (powerpc, powerpc64)
Approved by:	re (bz)

1 files changed, 25 insertions, 15 deletions

diff --git a/sys/vm/vm_page.h b/sys/vm/vm_page.h
index 1dda1e2..5431d79 100644
--- a/sys/vm/vm_page.h
+++ b/sys/vm/vm_page.h
@@ -125,12 +125,13 @@ struct vm_page {
 	struct md_page md;		/* machine dependant stuff */
 	uint8_t	queue;			/* page queue index (P,Q) */
 	int8_t segind;
-	u_short	flags;			/* see below */
+	short hold_count;		/* page hold count (P) */
 	uint8_t	order;			/* index of the buddy queue */
 	uint8_t pool;
 	u_short cow;			/* page cow mapping count (P) */
 	u_int wire_count;		/* wired down maps refs (P) */
-	short hold_count;		/* page hold count (P) */
+	uint8_t aflags;			/* access is atomic */
+	uint8_t flags;			/* see below, often immutable after alloc */
 	u_short oflags;			/* page flags (O) */
 	u_char	act_count;		/* page usage count (O) */
 	u_char	busy;			/* page busy count (O) */
@@ -225,21 +226,29 @@ extern struct vpglocks pa_lock[];
 /*
  * These are the flags defined for vm_page.
  *
- * PG_REFERENCED may be cleared only if the object containing the page is
+ * aflags are updated by atomic accesses. Use the vm_page_aflag_set()
+ * and vm_page_aflag_clear() functions to set and clear the flags.
+ *
+ * PGA_REFERENCED may be cleared only if the object containing the page is
  * locked.
  *
- * PG_WRITEABLE is set exclusively on managed pages by pmap_enter().  When it
+ * PGA_WRITEABLE is set exclusively on managed pages by pmap_enter().  When it
  * does so, the page must be VPO_BUSY.
  */
-#define	PG_CACHED	0x0001		/* page is cached */
-#define	PG_FREE		0x0002		/* page is free */
-#define PG_WINATCFLS	0x0004		/* flush dirty page on inactive q */
-#define	PG_FICTITIOUS	0x0008		/* physical page doesn't exist (O) */
-#define	PG_WRITEABLE	0x0010		/* page is mapped writeable */
-#define	PG_ZERO		0x0040		/* page is zeroed */
-#define PG_REFERENCED	0x0080		/* page has been referenced */
-#define PG_MARKER	0x1000		/* special queue marker page */
-#define	PG_SLAB		0x2000		/* object pointer is actually a slab */
+#define	PGA_WRITEABLE	0x01		/* page may be mapped writeable */
+#define	PGA_REFERENCED	0x02		/* page has been referenced */
+
+/*
+ * Page flags.  If changed at any other time than page allocation or
+ * freeing, the modification must be protected by the vm_page lock.
+ */
+#define	PG_CACHED	0x01		/* page is cached */
+#define	PG_FREE		0x02		/* page is free */
+#define	PG_FICTITIOUS	0x04		/* physical page doesn't exist (O) */
+#define	PG_ZERO		0x08		/* page is zeroed */
+#define	PG_MARKER	0x10		/* special queue marker page */
+#define	PG_SLAB		0x20		/* object pointer is actually a slab */
+#define	PG_WINATCFLS	0x40		/* flush dirty page on inactive q */
 
 /*
  * Misc constants.
@@ -341,8 +350,8 @@ extern struct vpglocks vm_page_queue_lock;
 #define	VM_ALLOC_COUNT_SHIFT	16
 #define	VM_ALLOC_COUNT(count)	((count) << VM_ALLOC_COUNT_SHIFT)
 
-void vm_page_flag_set(vm_page_t m, unsigned short bits);
-void vm_page_flag_clear(vm_page_t m, unsigned short bits);
+void vm_page_aflag_set(vm_page_t m, uint8_t bits);
+void vm_page_aflag_clear(vm_page_t m, uint8_t bits);
 void vm_page_busy(vm_page_t m);
 void vm_page_flash(vm_page_t m);
 void vm_page_io_start(vm_page_t m);
@@ -377,6 +386,7 @@ vm_page_t vm_page_next(vm_page_t m);
 int vm_page_pa_tryrelock(pmap_t, vm_paddr_t, vm_paddr_t *);
 vm_page_t vm_page_prev(vm_page_t m);
 void vm_page_putfake(vm_page_t m);
+void vm_page_reference(vm_page_t m);
 void vm_page_remove (vm_page_t);
 void vm_page_rename (vm_page_t, vm_object_t, vm_pindex_t);
 void vm_page_requeue(vm_page_t m);