[PATCH] Implement read_page_state

struct page_state is large (148 bytes) and we put them on the stack in awkward
code paths (page reclaim...)

So implement a simple read_page_state() which can be used to pluck out a
single member from the all-cpus page_state accumulators.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/page-flags.h

@@ -139,6 +139,10 @@ DECLARE_PER_CPU(struct page_state, page_states);
 
 extern void get_page_state(struct page_state *ret);
 extern void get_full_page_state(struct page_state *ret);
+extern unsigned long __read_page_state(unsigned offset);
+
+#define read_page_state(member) \
+	__read_page_state(offsetof(struct page_state, member))
 
 #define mod_page_state(member, delta)					\
 	do {								\

mm/page_alloc.c

@@ -988,6 +988,23 @@ void get_full_page_state(struct page_state *ret)
 	__get_page_state(ret, sizeof(*ret) / sizeof(unsigned long));
 }
 
+unsigned long __read_page_state(unsigned offset)
+{
+	unsigned long ret = 0;
+	int cpu;
+
+	for (cpu = 0; cpu < NR_CPUS; cpu++) {
+		unsigned long in;
+
+		if (!cpu_possible(cpu))
+			continue;
+
+		in = (unsigned long)&per_cpu(page_states, cpu) + offset;
+		ret += *((unsigned long *)in);
+	}
+	return ret;
+}
+
 void get_zone_counts(unsigned long *active,
 		unsigned long *inactive, unsigned long *free)
 {