mm/vmscan: replace implicit RECLAIM_ZONE checks with explicit checks

RECLAIM_ZONE was assumed to be unused because it was never explicitly
used in the kernel.  However, there were a number of places where it was
checked implicitly by checking 'node_reclaim_mode' for a zero value.

These zero checks are not great because it is not obvious what a zero
mode *means* in the code.  Replace them with a helper which makes it
more obvious: node_reclaim_enabled().

This helper also provides a handy place to explicitly check the
RECLAIM_ZONE bit itself.  Check it explicitly there to make it more
obvious where the bit can affect behavior.

This should have no functional impact.

Link: https://lkml.kernel.org/r/20210219172559.BF589C44@viggo.jf.intel.com
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Ben Widawsky <ben.widawsky@intel.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Acked-by: Christoph Lameter <cl@linux.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Alex Shi <alex.shi@linux.alibaba.com>
Cc: "Tobin C. Harding" <tobin@kernel.org>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Qian Cai <cai@lca.pw>
Cc: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/swap.h

@@ -12,6 +12,7 @@
 #include <linux/fs.h>
 #include <linux/atomic.h>
 #include <linux/page-flags.h>
+#include <uapi/linux/mempolicy.h>
 #include <asm/page.h>
 
 struct notifier_block;
@@ -378,6 +379,12 @@ extern int sysctl_min_slab_ratio;
 #define node_reclaim_mode 0
 #endif
 
+static inline bool node_reclaim_enabled(void)
+{
+	/* Is any node_reclaim_mode bit set? */
+	return node_reclaim_mode & (RECLAIM_ZONE|RECLAIM_WRITE|RECLAIM_UNMAP);
+}
+
 extern void check_move_unevictable_pages(struct pagevec *pvec);
 
 extern int kswapd_run(int nid);

mm/khugepaged.c

@@ -809,7 +809,7 @@ static bool khugepaged_scan_abort(int nid)
 	 * If node_reclaim_mode is disabled, then no extra effort is made to
 	 * allocate memory locally.
 	 */
-	if (!node_reclaim_mode)
+	if (!node_reclaim_enabled())
 		return false;
 
 	/* If there is a count for this node already, it must be acceptable */

mm/page_alloc.c

@@ -3968,7 +3968,7 @@ retry:
 			if (alloc_flags & ALLOC_NO_WATERMARKS)
 				goto try_this_zone;
 
-			if (node_reclaim_mode == 0 ||
+			if (!node_reclaim_enabled() ||
 			    !zone_allows_reclaim(ac->preferred_zoneref->zone, zone))
 				continue;