linux-kernel - Re: [PATCH v2 1/2] kernel/fork.c: get totalram_pages from memblock to calculate max

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Date: Mon, 1 Jul 2024 21:26:11 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Wei Yang <richard.weiyang@...il.com>
Cc: brauner@...nel.org, oleg@...hat.com, mjguzik@...il.com,
 tandersen@...flix.com, linux-kernel@...r.kernel.org, Mike Rapoport
 <rppt@...nel.org>, David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v2 1/2] kernel/fork.c: get totalram_pages from memblock
 to calculate max_threads

On Mon,  1 Jul 2024 01:34:09 +0000 Wei Yang <richard.weiyang@...il.com> wrote:

> Since we plan to move the accounting into __free_pages_core(),
> totalram_pages may not represent the total usable pages on system
> at this point when defer_init is enabled.

Yes, things like totalram_pages() are very old, and were a good idea at the
time, but things moved on.

> Instead we can get the total usable pages from memblock directly.
> 
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -44,6 +44,7 @@
>  #include <linux/fs.h>
>  #include <linux/mm.h>
>  #include <linux/mm_inline.h>
> +#include <linux/memblock.h>
>  #include <linux/nsproxy.h>
>  #include <linux/capability.h>
>  #include <linux/cpu.h>
> @@ -999,7 +1000,7 @@ void __init __weak arch_task_cache_init(void) { }
>  static void set_max_threads(unsigned int max_threads_suggested)
>  {
>  	u64 threads;
> -	unsigned long nr_pages = totalram_pages();
> +	unsigned long nr_pages = PHYS_PFN(memblock_phys_mem_size() - memblock_reserved_size());

The result of this subtraction has meaning.  Even if it is only used
once, I suspect it should be in a standalone function which has
documentation which describes that meaning.  Having fork.c make an
open-coded poke into memblock internals seems wrong, no?