Linux 4.8+ compatibility fix for vm stats

vm_node_stat must be used instead of vm_zone_stat. Unfortunately the
old code still compiles potentially leading to silent failure of
arc_evictable_memory()

AKAMAI: CR 3816601: Regression in zfs dropcache test

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Chunwei Chen <[email protected]>
Signed-off-by: Debabrata Banerjee <[email protected]>
Closes openzfs#6528

config/kernel-vm_node_stat.m4

@@ -0,0 +1,22 @@
+dnl #
+dnl # 4.8 API change
+dnl # kernel vm counters change
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_VM_NODE_STAT], [
+	AC_MSG_CHECKING([whether to use vm_node_stat based fn's])
+	ZFS_LINUX_TRY_COMPILE([
+		#include <linux/mm.h>
+		#include <linux/vmstat.h>
+        ],[
+			int a __attribute__ ((unused)) = NR_VM_NODE_STAT_ITEMS;
+			long x __attribute__ ((unused)) =
+				atomic_long_read(&vm_node_stat[0]);
+			(void) global_node_page_state(0);
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(ZFS_GLOBAL_NODE_PAGE_STATE, 1,
+			[using global_node_page_state()])
+	],[
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -120,6 +120,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_RENAME_WANTS_FLAGS
 	ZFS_AC_KERNEL_HAVE_GENERIC_SETXATTR
 	ZFS_AC_KERNEL_CURRENT_TIME
+	ZFS_AC_KERNEL_VM_NODE_STAT
 
 	AS_IF([test "$LINUX_OBJ" != "$LINUX"], [
 		KERNELMAKE_PARAMS="$KERNELMAKE_PARAMS O=$LINUX_OBJ"

module/zfs/arc.c

@@ -5064,8 +5064,13 @@ arc_evictable_memory(void)
 	 * Scale reported evictable memory in proportion to page cache, cap
 	 * at specified min/max.
 	 */
+#ifdef ZFS_GLOBAL_NODE_PAGE_STATE
+	uint64_t min = (ptob(global_node_page_state(NR_FILE_PAGES)) / 100) *
+	    zfs_arc_pc_percent;
+#else
 	uint64_t min = (ptob(global_page_state(NR_FILE_PAGES)) / 100) *
 	    zfs_arc_pc_percent;
+#endif
 	min = MAX(arc_c_min, MIN(arc_c_max, min));
 
 	if (arc_dirty >= min)