mm: replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion

Replace repeated (20 - PAGE_SHIFT) calculations with standard macros:
- MB_TO_PAGES(mb) converts MB to page count
- PAGES_TO_MB(pages) converts pages to MB

No functional change.

[[e-mail]: remove arc's private PAGES_TO_MB, remove its unused PAGES_TO_KB]
[[e-mail]: don't include mm.h due to include file ordering mess]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ye Liu <[e-mail]>
Acked-by: Zi Yan <[e-mail]>
Reviewed-by: Lorenzo Stoakes <[e-mail]>
Reviewed-by: Dev Jain <[e-mail]>
Acked-by: David Hildenbrand <[e-mail]>
Acked-by: Chris Li <[e-mail]>
Cc: Baolin Wang <[e-mail]>
Cc: Baoquan He <[e-mail]>
Cc: Barry Song <[e-mail]>
Cc: Ben Segall <[e-mail]>
Cc: Boqun Feng <[e-mail]>
Cc: Davidlohr Bueso <[e-mail]>
Cc: Dietmar Eggemann <[e-mail]>
Cc: Frederic Weisbecker <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Josh Triplett <[e-mail]>
Cc: Juri Lelli <[e-mail]>
Cc: Kairui Song <[e-mail]>
Cc: Kemeng Shi <[e-mail]>
Cc: Lai jiangshan <[e-mail]>
Cc: Liam Howlett <[e-mail]>
Cc: Mariano Pache <[e-mail]>
Cc: Mathieu Desnoyers <[e-mail]>
Cc: Mel Gorman <mgorman <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Mike Rapoport <[e-mail]>
Cc: Neeraj Upadhyay <[e-mail]>
Cc: Nhat Pham <[e-mail]>
Cc: "Paul E . McKenney" <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Ryan Roberts <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Cc: Suren Baghdasaryan <[e-mail]>
Cc: "Uladzislau Rezki (Sony)" <[e-mail]>
Cc: Valentin Schneider <[e-mail]>
Cc: Vincent Guittot <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

arch/arc/include/asm/arcregs.h

···
 /* Helpers */
 #define TO_KB(bytes)		((bytes) >> 10)
 #define TO_MB(bytes)		(TO_KB(bytes) >> 10)
-#define PAGES_TO_KB(n_pages)	((n_pages) << (PAGE_SHIFT - 10))
-#define PAGES_TO_MB(n_pages)	(PAGES_TO_KB(n_pages) >> 10)
-
 
 /*
  ***************************************************************

include/linux/mm.h

···
 
 extern void * high_memory;
 
+/*
+ * Convert between pages and MB
+ * 20 is the shift for 1MB (2^20 = 1MB)
+ * PAGE_SHIFT is the shift for page size (e.g., 12 for 4KB pages)
+ * So (20 - PAGE_SHIFT) converts between pages and MB
+ */
+#define PAGES_TO_MB(pages) ((pages) >> (20 - PAGE_SHIFT))
+#define MB_TO_PAGES(mb)    ((mb) << (20 - PAGE_SHIFT))
+
 #ifdef CONFIG_SYSCTL
 extern int sysctl_legacy_va_layout;
 #else

kernel/rcu/rcuscale.c

···
 		pr_alert("Total time taken by all kfree'ers: %llu ns, loops: %d, batches: %ld, memory footprint: %lldMB\n",
 		       (unsigned long long)(end_time - start_time), kfree_loops,
 		       rcuscale_seq_diff(b_rcu_gp_test_finished, b_rcu_gp_test_started),
-		       (mem_begin - mem_during) >> (20 - PAGE_SHIFT));
+		       PAGES_TO_MB(mem_begin - mem_during));
 
 		if (shutdown) {
 			smp_mb(); /* Assign before wake. */

kernel/sched/fair.c

···
 	 * by the PTE scanner and NUMA hinting faults should be trapped based
 	 * on resident pages
 	 */
-	nr_scan_pages = sysctl_numa_balancing_scan_size << (20 - PAGE_SHIFT);
+	nr_scan_pages = MB_TO_PAGES(sysctl_numa_balancing_scan_size);
 	rss = get_mm_rss(p->mm);
 	if (!rss)
 		rss = nr_scan_pages;
···
 		}
 
 		def_th = sysctl_numa_balancing_hot_threshold;
-		rate_limit = sysctl_numa_balancing_promote_rate_limit << \
-			(20 - PAGE_SHIFT);
+		rate_limit = MB_TO_PAGES(sysctl_numa_balancing_promote_rate_limit);
 		numa_promotion_adjust_threshold(pgdat, rate_limit, def_th);
 
 		th = pgdat->nbp_threshold ? : def_th;

mm/backing-dev.c

···
 /*
  * Initial write bandwidth: 100 MB/s
  */
-#define INIT_BW		(100 << (20 - PAGE_SHIFT))
+#define INIT_BW		MB_TO_PAGES(100)
 
 static int wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi,
 		   gfp_t gfp)

mm/huge_memory.c

···
 	 * where the extra memory used could hurt more than TLB overhead
 	 * is likely to save.  The admin can still enable it through /sys.
 	 */
-	if (totalram_pages() < (512 << (20 - PAGE_SHIFT))) {
+	if (totalram_pages() < MB_TO_PAGES(512)) {
 		transparent_hugepage_flags = 0;
 		return 0;
 	}

mm/swap.c

···
  */
 void __init swap_setup(void)
 {
-	unsigned long megs = totalram_pages() >> (20 - PAGE_SHIFT);
+	unsigned long megs = PAGES_TO_MB(totalram_pages());
 
 	/* Use a smaller cluster for small-memory machines */
 	if (megs < 16)