commit f4cb78af91e3 ("mm: add system wide stats items category") and
commit 9d8573111024 ("mm: don't account memmap per-node") renamed
NR_VM_WRITEBACK_STAT_ITEMS to NR_VM_STAT_ITEMS to track
memmap/memmap_boot pages system wide.

Extend the implementation so that the system wide page statistics can
be tracked using a generic interface. This patch is in preparation for
the next patch which adds a rarely modified system wide vmstat.

Note that this implementation uses global atomic fields with no per-cpu
optimizations as the existing usecase (memmap pages) is rarely modified
as well.

Signed-off-by: Kinsey Ho <kinseyho@google.com>
---
 include/linux/vmstat.h |  8 ++++++++
 mm/vmstat.c            | 32 +++++++++++++++++++++++---------
 2 files changed, 31 insertions(+), 9 deletions(-)

diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
index d2761bf8ff32..ac4d42c4fabd 100644
--- a/include/linux/vmstat.h
+++ b/include/linux/vmstat.h
@@ -145,6 +145,11 @@ extern atomic_long_t vm_zone_stat[NR_VM_ZONE_STAT_ITEMS];
 extern atomic_long_t vm_node_stat[NR_VM_NODE_STAT_ITEMS];
 extern atomic_long_t vm_numa_event[NR_VM_NUMA_EVENT_ITEMS];
 
+/*
+ * Global page accounting (no per cpu differentials).
+ */
+extern atomic_long_t vm_global_stat[NR_VM_STAT_ITEMS];
+
 #ifdef CONFIG_NUMA
 static inline void zone_numa_event_add(long x, struct zone *zone,
 				enum numa_stat_item item)
@@ -491,6 +496,9 @@ static inline void node_stat_sub_folio(struct folio *folio,
 	mod_node_page_state(folio_pgdat(folio), item, -folio_nr_pages(folio));
 }
 
+void mod_global_page_state(enum vm_stat_item item, long nr);
+unsigned long global_page_state(enum vm_stat_item item);
+
 extern const char * const vmstat_text[];
 
 static inline const char *zone_stat_name(enum zone_stat_item item)
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 22a294556b58..e5a6dd5106c2 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -161,9 +161,11 @@ void vm_events_fold_cpu(int cpu)
  */
 atomic_long_t vm_zone_stat[NR_VM_ZONE_STAT_ITEMS] __cacheline_aligned_in_smp;
 atomic_long_t vm_node_stat[NR_VM_NODE_STAT_ITEMS] __cacheline_aligned_in_smp;
+atomic_long_t vm_global_stat[NR_VM_STAT_ITEMS] __cacheline_aligned_in_smp;
 atomic_long_t vm_numa_event[NR_VM_NUMA_EVENT_ITEMS] __cacheline_aligned_in_smp;
 EXPORT_SYMBOL(vm_zone_stat);
 EXPORT_SYMBOL(vm_node_stat);
+EXPORT_SYMBOL(vm_global_stat);
 
 #ifdef CONFIG_NUMA
 static void fold_vm_zone_numa_events(struct zone *zone)
@@ -1033,22 +1035,34 @@ unsigned long node_page_state(struct pglist_data *pgdat,
 }
 #endif
 
+void mod_global_page_state(enum vm_stat_item item, long nr)
+{
+	atomic_long_add(nr, &vm_global_stat[item]);
+}
+
+unsigned long global_page_state(enum vm_stat_item item)
+{
+	long x = atomic_long_read(&vm_global_stat[item]);
+#ifdef CONFIG_SMP
+	if (x < 0)
+		x = 0;
+#endif
+	return x;
+}
+
 /*
  * Count number of pages "struct page" and "struct page_ext" consume.
- * nr_memmap_boot_pages: # of pages allocated by boot allocator
- * nr_memmap_pages: # of pages that were allocated by buddy allocator
+ * NR_MEMMAP_BOOT_PAGES: # of pages allocated by boot allocator
+ * NR_MEMMAP_PAGES: # of pages that were allocated by buddy allocator
  */
-static atomic_long_t nr_memmap_boot_pages = ATOMIC_LONG_INIT(0);
-static atomic_long_t nr_memmap_pages = ATOMIC_LONG_INIT(0);
-
 void memmap_boot_pages_add(long delta)
 {
-	atomic_long_add(delta, &nr_memmap_boot_pages);
+	mod_global_page_state(NR_MEMMAP_BOOT_PAGES, delta);
 }
 
 void memmap_pages_add(long delta)
 {
-	atomic_long_add(delta, &nr_memmap_pages);
+	mod_global_page_state(NR_MEMMAP_PAGES, delta);
 }
 
 #ifdef CONFIG_COMPACTION
@@ -1880,8 +1894,8 @@ static void *vmstat_start(struct seq_file *m, loff_t *pos)
 
 	global_dirty_limits(v + NR_DIRTY_BG_THRESHOLD,
 			    v + NR_DIRTY_THRESHOLD);
-	v[NR_MEMMAP_PAGES] = atomic_long_read(&nr_memmap_pages);
-	v[NR_MEMMAP_BOOT_PAGES] = atomic_long_read(&nr_memmap_boot_pages);
+	for (int i = NR_MEMMAP_PAGES; i < NR_VM_STAT_ITEMS; i++)
+		v[i] = global_page_state(i);
 	v += NR_VM_STAT_ITEMS;
 
 #ifdef CONFIG_VM_EVENT_COUNTERS
-- 
2.47.0.163.g1226f6d8fa-goog