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

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Message-ID: <416948e5-6308-4d57-8a05-d611522f5f2e@lucifer.local>
Date: Fri, 18 Jul 2025 10:57:36 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Ye Liu <ye.liu@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
        Joel Fernandes <joelagnelf@...dia.com>,
        Boqun Feng <boqun.feng@...il.com>, Uladzislau Rezki <urezki@...il.com>,
        Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ye Liu <liuye@...inos.cn>, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>, Zqiang <qiang.zhang@...ux.dev>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Valentin Schneider <vschneid@...hat.com>, Zi Yan <ziy@...dia.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
        Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
        Kemeng Shi <shikemeng@...weicloud.com>,
        Kairui Song <kasong@...cent.com>, Nhat Pham <nphamcs@...il.com>,
        Baoquan He <bhe@...hat.com>, Chris Li <chrisl@...nel.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org, rcu@...r.kernel.org
Subject: Re: [PATCH] mm: Replace (20 - PAGE_SHIFT) with common macros for
 pages<->MB conversion

On Fri, Jul 18, 2025 at 10:41:32AM +0800, Ye Liu wrote:
> From: Ye Liu <liuye@...inos.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@...inos.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@...cle.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
>