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

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Message-Id: <20250717202146.0b9b8f59eae21da8529af559@linux-foundation.org>
Date: Thu, 17 Jul 2025 20:21:46 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Ye Liu <ye.liu@...ux.dev>
Cc: 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>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 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, 18 Jul 2025 10:41:32 +0800 Ye Liu <ye.liu@...ux.dev> 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.
> 
> ...
>
> +/*
> + * 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.