Re: [PATCH 08/12] mm/sparse: move __highest_used_section_nr handling

["David Hildenbrand (Arm)" <david@kernel.org>]

On 9/10/26 16:16, Lorenzo Stoakes (ARM) wrote:
> On Wed, Sep 09, 2026 at 03:33:01PM +0200, David Hildenbrand (Arm) wrote:
>> Let's move it to sparse_init_one_section(). However, to keep early
>> boot processing working, we also have to initialize it in
>> sparse_sections_init().
> 
> A why might be nice :)

Agreed, I'll mention that.

> 
> I guess preparing for removal of __section_mark_present()?
> 

Right intuition :)

> Also good to have arguments as to why this is equivalent of previous
> behaviour.
> 
> E.g. higher pfn = higher section nr so naturally the highest is the one you
> end up wtih at the end of sparse_sections_init()?

We go over all sections, just earlier.

> 
>>
>> Should we use READ_ONCE/WRITE_ONCE with __highest_used_section_nr?
>> Probably, something for another day.
> 
> It might be worth expanding this a bit. Do multiple threads read/write this
> concurrently?

Hah, I'll probably just drop it. I was just stumbling over readers vs.
concurrent updates and thought "that looks suspicious".

> 
> No functional change intended here or is one intended? :)
> 

Certainly no change intended ;)

> Before it was:
> 
> mm_core_init_early() -> sparse_sections_init() -> __section_mark_present()
>                           sparse_add_section() -> __section_mark_present()
> 
> Now:
> 
>                                                  mm_core_init_early() -> sparse_sections_init() [early]
>                                                  sparse_add_section() -> sparse_init_one_section()
> 
> But also called from mm_core_init_early():
> 
> sparse_init() -> sparse_metadata_init() -> sparse_metadata_init_nid() -> sparse_init_one_section()
> 
> Are both required?

sparse_metadata_init() relies on __highest_used_section_nr in the
for_each_early_section_nr / for_each_present_section_nr, so it is required.

I could probable move the update on the hotplug side into sparse_add_section()
instead!

> 
> 
>>
>> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
>> ---
>>  mm/sparse.c | 5 +++--
>>  mm/sparse.h | 6 +++---
>>  2 files changed, 6 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/sparse.c b/mm/sparse.c
>> index 2b41ae36f20b8..2d0f2db34f4cf 100644
>> --- a/mm/sparse.c
>> +++ b/mm/sparse.c
>> @@ -177,7 +177,7 @@ static inline unsigned long first_present_section_nr(void)
>>
>>  void __init sparse_sections_init(void)
>>  {
>> -	unsigned long pfn, start_pfn, end_pfn;
>> +	unsigned long pfn, start_pfn, end_pfn, section_nr;
>>  	int i, nid;
>>
>>  	sparse_extreme_init();
>> @@ -187,9 +187,9 @@ void __init sparse_sections_init(void)
>>  		mminit_validate_memmodel_limits(&start_pfn, &end_pfn);
>>
>>  		for (pfn = start_pfn; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
>> -			unsigned long section_nr = pfn_to_section_nr(pfn);
>>  			struct mem_section *ms;
>>
>> +			section_nr = pfn_to_section_nr(pfn);
>>  			sparse_index_init(section_nr, nid);
>>  			ms = __nr_to_section(section_nr);
>>  			if (ms->section_mem_map)
>> @@ -201,6 +201,7 @@ void __init sparse_sections_init(void)
>>  			__section_mark_present(ms, section_nr);
>>  		}
>>  	}
>> +	__highest_used_section_nr = section_nr;
>>  }
>>
>>  #ifndef CONFIG_SPARSEMEM_VMEMMAP
>> diff --git a/mm/sparse.h b/mm/sparse.h
>> index 7c5d82ceb7142..a3af4967fd5c5 100644
>> --- a/mm/sparse.h
>> +++ b/mm/sparse.h
>> @@ -97,6 +97,9 @@ static inline void sparse_init_one_section(struct mem_section *ms,
>>
>>  	BUILD_BUG_ON(SECTION_MAP_LAST_BIT > PFN_SECTION_SHIFT);
>>
>> +	if (section_nr > __highest_used_section_nr)
>> +		__highest_used_section_nr = section_nr;
>> +
> 
> Could also be:
> 
> 	section_nr = max(section_nr, __highest_used_section_nr);

Ack!

-- 
Cheers,

David