[PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion

[PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion

[Ye Liu]

From: Ye Liu <liuye@kylinos.cn>

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.

Signed-off-by: Ye Liu <liuye@kylinos.cn>
---
 include/linux/mm.h    | 9 +++++++++
 kernel/rcu/rcuscale.c | 2 +-
 kernel/sched/fair.c   | 5 ++---
 mm/backing-dev.c      | 2 +-
 mm/huge_memory.c      | 2 +-
 mm/swap.c             | 2 +-
 6 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 957acde6ae62..0c1b2c074142 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 b9b4bbbf0af6..ae1d9a7ef202 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1489,7 +1489,7 @@ 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;
@@ -1926,8 +1926,7 @@ 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 389620c65a5f..dcc33d9c300f 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)
-- 
2.43.0

Re: [PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion

[Andrew Morton]

On Fri, 18 Jul 2025 10:41:32 +0800 Ye Liu <ye.liu@linux.dev> wrote:

> From: Ye Liu <liuye@kylinos.cn>
> 
> 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.
> 
> ...
>
> +/*
> + * 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
>
> ...
>
> @@ -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. */

But, but, but, obscure hard-coded magic numbers are there for our job
security!

Oh well, we got caught.  Applied, thanks.

Re: [PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion

[David Hildenbrand]

On 18.07.25 04:41, Ye Liu wrote:
> From: Ye Liu <liuye@kylinos.cn>
> 
> 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.
> 
> Signed-off-by: Ye Liu <liuye@kylinos.cn>
> ---

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb

Re: [PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion

[Dev Jain]

On 18/07/25 8:11 am, Ye Liu wrote:
> From: Ye Liu <liuye@kylinos.cn>
>
> 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.
>
> Signed-off-by: Ye Liu <liuye@kylinos.cn>
> ---
>   

Reviewed-by: Dev Jain <dev.jain@arm.com>

Re: [PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion

[Dev Jain]

On 18/07/25 8:11 am, Ye Liu wrote:
> From: Ye Liu <liuye@kylinos.cn>
>
> 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.
>
> Signed-off-by: Ye Liu <liuye@kylinos.cn>
> ---
>   

sh and x86 have their own pages_to_mb, drivers/target/target_core_user.c too.
I guess no one likes to clean the kernel :)

Re: [PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion

[Lorenzo Stoakes]

On Fri, Jul 18, 2025 at 03:16:55PM +0530, Dev Jain wrote:
>
> On 18/07/25 8:11 am, Ye Liu wrote:
> > From: Ye Liu <liuye@kylinos.cn>
> >
> > 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.
> >
> > Signed-off-by: Ye Liu <liuye@kylinos.cn>
> > ---
>
> sh and x86 have their own pages_to_mb, drivers/target/target_core_user.c too.
> I guess no one likes to clean the kernel :)
>

*A wild Lorenzo appaers*

Re: [PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion

[Lorenzo Stoakes]

On Fri, Jul 18, 2025 at 10:41:32AM +0800, Ye Liu wrote:
> From: Ye Liu <liuye@kylinos.cn>
>
> 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.
>
> Signed-off-by: Ye Liu <liuye@kylinos.cn>

Nice idea :)

NOte I see arch/x86/include/asm/pgtable.h has a pages_to_mb() static inline
declaration, but probably being an asm include can't ref mm.h so meh not a big
deal.

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  include/linux/mm.h    | 9 +++++++++
>  kernel/rcu/rcuscale.c | 2 +-
>  kernel/sched/fair.c   | 5 ++---
>  mm/backing-dev.c      | 2 +-
>  mm/huge_memory.c      | 2 +-
>  mm/swap.c             | 2 +-
>  6 files changed, 15 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 957acde6ae62..0c1b2c074142 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 b9b4bbbf0af6..ae1d9a7ef202 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1489,7 +1489,7 @@ 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;
> @@ -1926,8 +1926,7 @@ 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 389620c65a5f..dcc33d9c300f 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)
> --
> 2.43.0
>

Re: [PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion

[Matthew Wilcox]

On Fri, Jul 18, 2025 at 10:57:36AM +0100, Lorenzo Stoakes wrote:
> NOte I see arch/x86/include/asm/pgtable.h has a pages_to_mb() static inline
> declaration, but probably being an asm include can't ref mm.h so meh not a big
> deal.

Should probably go to linux/sizes.h, except that it uses PAGE_SIZE which
isn't available there.  But asm-generic/getorder.h might be a good place
for it.

(hm, is including getorder.h safe by itself?  looks like it relies on
something else to bring in the definition of PAGE_SHIFT)

Re: [PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion

[Zi Yan]

On 17 Jul 2025, at 22:41, Ye Liu wrote:

> From: Ye Liu <liuye@kylinos.cn>
>
> 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.
>
> Signed-off-by: Ye Liu <liuye@kylinos.cn>
> ---
>  include/linux/mm.h    | 9 +++++++++
>  kernel/rcu/rcuscale.c | 2 +-
>  kernel/sched/fair.c   | 5 ++---
>  mm/backing-dev.c      | 2 +-
>  mm/huge_memory.c      | 2 +-
>  mm/swap.c             | 2 +-
>  6 files changed, 15 insertions(+), 7 deletions(-)
>

Thanks.

Acked-by: Zi Yan <ziy@nvidia.com>

--
Best Regards,
Yan, Zi

Re: [PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion

[Chris Li]

On Thu, Jul 17, 2025 at 7:42 PM Ye Liu <ye.liu@linux.dev> wrote:
>
> From: Ye Liu <liuye@kylinos.cn>
>
> 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.

Thanks for doing this.

Acked-by: Chris Li <chrisl@kernel.org>

Chris

Re: [PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion

[kernel test robot]

Hi Ye,

kernel test robot noticed the following build warnings:

[auto build test WARNING on akpm-mm/mm-everything]

url:    https://github.com/intel-lab-lkp/linux/commits/Ye-Liu/mm-Replace-20-PAGE_SHIFT-with-common-macros-for-pages-MB-conversion/20250718-104347
base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link:    https://lore.kernel.org/r/20250718024134.1304745-1-ye.liu%40linux.dev
patch subject: [PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion
config: arc-randconfig-001-20250719 (https://download.01.org/0day-ci/archive/20250719/202507190319.0rqhQw5l-lkp@intel.com/config)
compiler: arc-linux-gcc (GCC) 14.3.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250719/202507190319.0rqhQw5l-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202507190319.0rqhQw5l-lkp@intel.com/

All warnings (new ones prefixed by >>):

   In file included from arch/arc/kernel/asm-offsets.c:7:
>> include/linux/mm.h:78:9: warning: "PAGES_TO_MB" redefined
      78 | #define PAGES_TO_MB(pages) ((pages) >> (20 - PAGE_SHIFT))
         |         ^~~~~~~~~~~
   In file included from arch/arc/include/asm/irqflags-arcv2.h:9,
                    from arch/arc/include/asm/irqflags.h:13,
                    from include/linux/irqflags.h:18,
                    from include/linux/spinlock.h:59,
                    from include/linux/sched.h:2205,
                    from arch/arc/kernel/asm-offsets.c:6:
   arch/arc/include/asm/arcregs.h:155:9: note: this is the location of the previous definition
     155 | #define PAGES_TO_MB(n_pages)    (PAGES_TO_KB(n_pages) >> 10)
         |         ^~~~~~~~~~~
--
   In file included from arch/arc/include/asm/cacheflush.h:18,
                    from include/linux/cacheflush.h:5,
                    from include/linux/highmem.h:8,
                    from include/linux/bvec.h:10,
                    from include/linux/blk_types.h:10,
                    from include/linux/blkdev.h:9,
                    from fs/ufs/super.c:83:
>> include/linux/mm.h:78:9: warning: "PAGES_TO_MB" redefined
      78 | #define PAGES_TO_MB(pages) ((pages) >> (20 - PAGE_SHIFT))
         |         ^~~~~~~~~~~
   In file included from arch/arc/include/asm/irqflags-arcv2.h:9,
                    from arch/arc/include/asm/irqflags.h:13,
                    from include/linux/irqflags.h:18,
                    from include/linux/spinlock.h:59,
                    from include/linux/mmzone.h:8,
                    from include/linux/gfp.h:7,
                    from include/linux/umh.h:4,
                    from include/linux/kmod.h:9,
                    from include/linux/module.h:17,
                    from fs/ufs/super.c:70:
   arch/arc/include/asm/arcregs.h:155:9: note: this is the location of the previous definition
     155 | #define PAGES_TO_MB(n_pages)    (PAGES_TO_KB(n_pages) >> 10)
         |         ^~~~~~~~~~~
   fs/ufs/super.c: In function 'ufs_reconfigure':
   fs/ufs/super.c:1246:22: warning: variable 'ufstype' set but not used [-Wunused-but-set-variable]
    1246 |         unsigned int ufstype;
         |                      ^~~~~~~
--
   In file included from arch/arc/kernel/asm-offsets.c:7:
>> include/linux/mm.h:78:9: warning: "PAGES_TO_MB" redefined
      78 | #define PAGES_TO_MB(pages) ((pages) >> (20 - PAGE_SHIFT))
         |         ^~~~~~~~~~~
   In file included from arch/arc/include/asm/irqflags-arcv2.h:9,
                    from arch/arc/include/asm/irqflags.h:13,
                    from include/linux/irqflags.h:18,
                    from include/linux/spinlock.h:59,
                    from include/linux/sched.h:2205,
                    from arch/arc/kernel/asm-offsets.c:6:
   arch/arc/include/asm/arcregs.h:155:9: note: this is the location of the previous definition
     155 | #define PAGES_TO_MB(n_pages)    (PAGES_TO_KB(n_pages) >> 10)
         |         ^~~~~~~~~~~


vim +/PAGES_TO_MB +78 include/linux/mm.h

    71	
    72	/*
    73	 * Convert between pages and MB
    74	 * 20 is the shift for 1MB (2^20 = 1MB)
    75	 * PAGE_SHIFT is the shift for page size (e.g., 12 for 4KB pages)
    76	 * So (20 - PAGE_SHIFT) converts between pages and MB
    77	 */
  > 78	#define PAGES_TO_MB(pages) ((pages) >> (20 - PAGE_SHIFT))
    79	#define MB_TO_PAGES(mb)    ((mb) << (20 - PAGE_SHIFT))
    80	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion

[Lorenzo Stoakes]

Ah yeah looks like this is already defined for arc only (!) in
arch/arc/include/asm/arcregs.h.

Maybe pop in a #ifndef PAGES_TO_MB ? It seems to be equivalent.

On Sat, Jul 19, 2025 at 04:06:04AM +0800, kernel test robot wrote:
> Hi Ye,
>
> kernel test robot noticed the following build warnings:
>
> [auto build test WARNING on akpm-mm/mm-everything]
>
> url:    https://github.com/intel-lab-lkp/linux/commits/Ye-Liu/mm-Replace-20-PAGE_SHIFT-with-common-macros-for-pages-MB-conversion/20250718-104347
> base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
> patch link:    https://lore.kernel.org/r/20250718024134.1304745-1-ye.liu%40linux.dev
> patch subject: [PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion
> config: arc-randconfig-001-20250719 (https://download.01.org/0day-ci/archive/20250719/202507190319.0rqhQw5l-lkp@intel.com/config)
> compiler: arc-linux-gcc (GCC) 14.3.0
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250719/202507190319.0rqhQw5l-lkp@intel.com/reproduce)
>
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202507190319.0rqhQw5l-lkp@intel.com/
>
> All warnings (new ones prefixed by >>):
>
>    In file included from arch/arc/kernel/asm-offsets.c:7:
> >> include/linux/mm.h:78:9: warning: "PAGES_TO_MB" redefined
>       78 | #define PAGES_TO_MB(pages) ((pages) >> (20 - PAGE_SHIFT))
>          |         ^~~~~~~~~~~
>    In file included from arch/arc/include/asm/irqflags-arcv2.h:9,
>                     from arch/arc/include/asm/irqflags.h:13,
>                     from include/linux/irqflags.h:18,
>                     from include/linux/spinlock.h:59,
>                     from include/linux/sched.h:2205,
>                     from arch/arc/kernel/asm-offsets.c:6:
>    arch/arc/include/asm/arcregs.h:155:9: note: this is the location of the previous definition
>      155 | #define PAGES_TO_MB(n_pages)    (PAGES_TO_KB(n_pages) >> 10)
>          |         ^~~~~~~~~~~
> --
>    In file included from arch/arc/include/asm/cacheflush.h:18,
>                     from include/linux/cacheflush.h:5,
>                     from include/linux/highmem.h:8,
>                     from include/linux/bvec.h:10,
>                     from include/linux/blk_types.h:10,
>                     from include/linux/blkdev.h:9,
>                     from fs/ufs/super.c:83:
> >> include/linux/mm.h:78:9: warning: "PAGES_TO_MB" redefined
>       78 | #define PAGES_TO_MB(pages) ((pages) >> (20 - PAGE_SHIFT))
>          |         ^~~~~~~~~~~
>    In file included from arch/arc/include/asm/irqflags-arcv2.h:9,
>                     from arch/arc/include/asm/irqflags.h:13,
>                     from include/linux/irqflags.h:18,
>                     from include/linux/spinlock.h:59,
>                     from include/linux/mmzone.h:8,
>                     from include/linux/gfp.h:7,
>                     from include/linux/umh.h:4,
>                     from include/linux/kmod.h:9,
>                     from include/linux/module.h:17,
>                     from fs/ufs/super.c:70:
>    arch/arc/include/asm/arcregs.h:155:9: note: this is the location of the previous definition
>      155 | #define PAGES_TO_MB(n_pages)    (PAGES_TO_KB(n_pages) >> 10)
>          |         ^~~~~~~~~~~
>    fs/ufs/super.c: In function 'ufs_reconfigure':
>    fs/ufs/super.c:1246:22: warning: variable 'ufstype' set but not used [-Wunused-but-set-variable]
>     1246 |         unsigned int ufstype;
>          |                      ^~~~~~~
> --
>    In file included from arch/arc/kernel/asm-offsets.c:7:
> >> include/linux/mm.h:78:9: warning: "PAGES_TO_MB" redefined
>       78 | #define PAGES_TO_MB(pages) ((pages) >> (20 - PAGE_SHIFT))
>          |         ^~~~~~~~~~~
>    In file included from arch/arc/include/asm/irqflags-arcv2.h:9,
>                     from arch/arc/include/asm/irqflags.h:13,
>                     from include/linux/irqflags.h:18,
>                     from include/linux/spinlock.h:59,
>                     from include/linux/sched.h:2205,
>                     from arch/arc/kernel/asm-offsets.c:6:
>    arch/arc/include/asm/arcregs.h:155:9: note: this is the location of the previous definition
>      155 | #define PAGES_TO_MB(n_pages)    (PAGES_TO_KB(n_pages) >> 10)
>          |         ^~~~~~~~~~~
>
>
> vim +/PAGES_TO_MB +78 include/linux/mm.h
>
>     71
>     72	/*
>     73	 * Convert between pages and MB
>     74	 * 20 is the shift for 1MB (2^20 = 1MB)
>     75	 * PAGE_SHIFT is the shift for page size (e.g., 12 for 4KB pages)
>     76	 * So (20 - PAGE_SHIFT) converts between pages and MB
>     77	 */
>   > 78	#define PAGES_TO_MB(pages) ((pages) >> (20 - PAGE_SHIFT))
>     79	#define MB_TO_PAGES(mb)    ((mb) << (20 - PAGE_SHIFT))
>     80
>
> --
> 0-DAY CI Kernel Test Service
> https://github.com/intel/lkp-tests/wiki

Re: [PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion

[Andrew Morton]

On Sat, 19 Jul 2025 07:56:16 +0100 Lorenzo Stoakes <lorenzo.stoakes@oracle.com> wrote:

> Ah yeah looks like this is already defined for arc only (!) in
> arch/arc/include/asm/arcregs.h.
> 
> Maybe pop in a #ifndef PAGES_TO_MB ? It seems to be equivalent.

Well, those arc helpers

/* 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)

are simply in the wrong place.

I was thinking move all four into mm.h (for the lack of a better
place).  Then someone can make a career out of hunting down all the code
sites which can use them.

eg,

hp2:/usr/src/linux-6.16-rc5> fgrep -r ">> 10" . | wc -l
886

Re: [PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion

[Lorenzo Stoakes]

On Sat, Jul 19, 2025 at 04:38:07PM -0700, Andrew Morton wrote:
> On Sat, 19 Jul 2025 07:56:16 +0100 Lorenzo Stoakes <lorenzo.stoakes@oracle.com> wrote:
>
> > Ah yeah looks like this is already defined for arc only (!) in
> > arch/arc/include/asm/arcregs.h.
> >
> > Maybe pop in a #ifndef PAGES_TO_MB ? It seems to be equivalent.
>
> Well, those arc helpers
>
> /* 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)
>
> are simply in the wrong place.
>
> I was thinking move all four into mm.h (for the lack of a better
> place).  Then someone can make a career out of hunting down all the code
> sites which can use them.
>
> eg,
>
> hp2:/usr/src/linux-6.16-rc5> fgrep -r ">> 10" . | wc -l
> 886

Yeah I agree for sure, this is misplaced, but maybe one for a follow up
patch?

There's more work to be done here, I already have an idea for a patch to
fix another similar case...

Re: [PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for pages<->MB conversion

[Andrew Morton]

On Sun, 20 Jul 2025 09:59:55 +0100 Lorenzo Stoakes <lorenzo.stoakes@oracle.com> wrote:

> > /* 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)
> >
> > are simply in the wrong place.
> >
> > I was thinking move all four into mm.h (for the lack of a better
> > place).  Then someone can make a career out of hunting down all the code
> > sites which can use them.
> >
> > eg,
> >
> > hp2:/usr/src/linux-6.16-rc5> fgrep -r ">> 10" . | wc -l
> > 886
> 
> Yeah I agree for sure, this is misplaced, but maybe one for a follow up
> patch?
> 
> There's more work to be done here, I already have an idea for a patch to
> fix another similar case...

OK.

I could put a couple of ifndefs in there to make this patch happy, but
this patch is just a little cleanup and it's a very small part of any
kernel-wide overhaul of these conversion operations.

So for now I think I'll retain this patch in mm-new to prevent this
linux-next build error.  I'll do this as a reminder until someone gets
down and addresses all this in a broader fashion.