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

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Date: Tue, 2 Jul 2024 09:02:31 +0200
From: David Hildenbrand <david@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>,
 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>
Subject: Re: [PATCH v2 1/2] kernel/fork.c: get totalram_pages from memblock to
 calculate max_threads

On 02.07.24 06:26, Andrew Morton wrote:
> 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?

I was just about the comment the same thing:

I'm starting to wonder if we should have a helper like

memblock_estimated_nr_pages()

or sth like that that abstracts this?

-- 
Cheers,

David / dhildenb