Re: [PATCH v3 1/3] mm/memblock: introduce a new helper memblock_estimated_nr_pages()

[David Hildenbrand <david@redhat.com>]

On 06.07.24 03:28, Wei Yang wrote:
> On Fri, Jul 05, 2024 at 12:09:48PM +0300, Mike Rapoport wrote:
>> On Wed, Jul 03, 2024 at 12:51:49AM +0000, Wei Yang wrote:
>>> Instead of using raw memblock api, we wrap a new helper for user.
>>
>> The changelog should be more elaborate and explain why this function is
>> needed.
>>
>>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>>> ---
>>>   include/linux/memblock.h |  1 +
>>>   mm/memblock.c            | 19 +++++++++++++++++++
>>>   2 files changed, 20 insertions(+)
>>>
>>> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
>>> index 40c62aca36ec..7d1c32b3dc12 100644
>>> --- a/include/linux/memblock.h
>>> +++ b/include/linux/memblock.h
>>> @@ -486,6 +486,7 @@ static inline __init_memblock bool memblock_bottom_up(void)
>>>   
>>>   phys_addr_t memblock_phys_mem_size(void);
>>>   phys_addr_t memblock_reserved_size(void);
>>> +unsigned long memblock_estimated_nr_pages(void);
>>>   phys_addr_t memblock_start_of_DRAM(void);
>>>   phys_addr_t memblock_end_of_DRAM(void);
>>>   void memblock_enforce_memory_limit(phys_addr_t memory_limit);
>>> diff --git a/mm/memblock.c b/mm/memblock.c
>>> index e81fb68f7f88..c1f1aac0459f 100644
>>> --- a/mm/memblock.c
>>> +++ b/mm/memblock.c
>>> @@ -1729,6 +1729,25 @@ phys_addr_t __init_memblock memblock_reserved_size(void)
>>>   	return memblock.reserved.total_size;
>>>   }
>>>   
>>> +/**
>>> + * memblock_estimated_nr_pages - return number of pages from memblock point of
>>> + * view
>>
>> This function returns the estimate for free pages, not the number of pages
>> in RAM.
>>
>> How about memblock_estimated_nr_free_pages()?
>>
>>> + * some calculation before all pages are freed to buddy system, especially
>>> + * when CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled.
>>
>> I'm failing to parse this sentence. The return value here won't depend on
>> CONFIG_DEFERRED_STRUCT_PAGE_INIT.
>>
>>> + *
>>> + * At this point, we can get this information from memblock. Since the system
>>> + * state is not settle down and address alignment, the value is an estimation.
>>> + *
>>> + * Return:
>>> + * An estimated number of pages from memblock point of view.
>>
>>                             ^ free
>>
>>> + */
>>> +unsigned long __init memblock_estimated_nr_pages(void)
>>> +{
>>> +	return PHYS_PFN(memblock_phys_mem_size() - memblock_reserved_size());
>>> +}
>>> +
>>>   /* lowest address */
>>>   phys_addr_t __init_memblock memblock_start_of_DRAM(void)
>>>   {
>>> -- 
>>> 2.34.1
>>>
> 
> Thanks for review. Is this one looks better?
> 
> Subject: [PATCH] mm/memblock: introduce a new helper memblock_estimated_nr_free_pages()
> 
> During bootup, system may need the number of free pages in the whole system
> to do some calculation before all pages are freed to buddy system. Usually
> this number is get from totalram_pages(). Since we plan to move the free
> pages accounting in __free_pages_core(), this value may not represent
> total free pages at the early stage, especially when
> CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled.
> 
> Instead of using raw memblock api, let's introduce a new helper for user
> to get the estimated number of free pages from memblock point of view.
> 
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> CC: David Hildenbrand <david@redhat.com>
> ---
>   include/linux/memblock.h |  1 +
>   mm/memblock.c            | 22 ++++++++++++++++++++++
>   2 files changed, 23 insertions(+)
> 
> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> index 40c62aca36ec..7d1c32b3dc12 100644
> --- a/include/linux/memblock.h
> +++ b/include/linux/memblock.h
> @@ -486,6 +486,7 @@ static inline __init_memblock bool memblock_bottom_up(void)
>   
>   phys_addr_t memblock_phys_mem_size(void);
>   phys_addr_t memblock_reserved_size(void);
> +unsigned long memblock_estimated_nr_pages(void);
>   phys_addr_t memblock_start_of_DRAM(void);
>   phys_addr_t memblock_end_of_DRAM(void);
>   void memblock_enforce_memory_limit(phys_addr_t memory_limit);
> diff --git a/mm/memblock.c b/mm/memblock.c
> index e81fb68f7f88..00decc42e02b 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -1729,6 +1729,28 @@ phys_addr_t __init_memblock memblock_reserved_size(void)
>   	return memblock.reserved.total_size;
>   }
>   
> +/**
> + * memblock_estimated_nr_free_pages - return estimated number of free pages
> + * from memblock point of view
> + *
> + * During bootup, system may need the number of free pages in the whole system
> + * to do some calculation before all pages are freed to buddy system. Usually
> + * this number is get from totalram_pages(). Since we plan to move the free
> + * pages accounting in __free_pages_core(), this value may not represent total
> + * free pages at the early stage, especially when > + * CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled.

These historical details should be dropped. "Since we plan ..." will 
easily get outdated soon.

* During bootup, subsystems might need a rough estimate of the number of 
* free pages in the whole system, before precise numbers are available
* from the buddy. Especially with CONFIG_DEFERRED_STRUCT_PAGE_INIT, the
* numbers obtained from the buddy might be very imprecise during bootup.

?

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

-- 
Cheers,

David / dhildenb