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.

[akpm@linux-foundation.org: remove arc's private PAGES_TO_MB, remove its unused PAGES_TO_KB]
[akpm@linux-foundation.org: don't include mm.h due to include file ordering mess]
Link: https://lkml.kernel.org/r/20250718024134.1304745-1-ye.liu@linux.dev
Signed-off-by: Ye Liu <liuye@kylinos.cn>
Acked-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Chris Li <chrisl@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Lai jiangshan <jiangshanlai@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Mel Gorman <mgorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: "Paul E . McKenney" <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

7 files changed, 15 insertions, 10 deletions

diff --git a/arch/arc/include/asm/arcregs.h b/arch/arc/include/asm/arcregs.h
index a31bbf5c8bbc..d84908a177bd 100644
--- a/arch/arc/include/asm/arcregs.h
+++ b/arch/arc/include/asm/arcregs.h
@@ -151,9 +151,6 @@
 /* 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)
-
 
 /*
  ***************************************************************
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1ae97a0b8ec7..b626d1bacef5 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -69,6 +69,15 @@ static inline void totalram_pages_add(long count)
 
 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
diff --git a/kernel/rcu/rcuscale.c b/kernel/rcu/rcuscale.c
index b521d0455992..7484d8ad5767 100644
--- a/kernel/rcu/rcuscale.c
+++ b/kernel/rcu/rcuscale.c
@@ -796,7 +796,7 @@ kfree_scale_thread(void *arg)
 		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. */
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 82c8d804c54c..e256793b9a08 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1495,7 +1495,7 @@ static unsigned int task_nr_scan_windows(struct task_struct *p)
 	 * 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;
@@ -1934,8 +1934,7 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio,
 		}
 
 		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;
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 783904d8c5ef..e4d578e6121c 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -510,7 +510,7 @@ static void wb_update_bandwidth_workfn(struct work_struct *work)
 /*
  * 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)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 9c38a95e9f09..2b4ea5a2ce7d 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -911,7 +911,7 @@ static int __init hugepage_init(void)
 	 * 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;
 	}
diff --git a/mm/swap.c b/mm/swap.c
index 3632dd061beb..cb164f9ef9e3 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -1096,7 +1096,7 @@ static const struct ctl_table swap_sysctl_table[] = {
  */
 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)