Re: [PATCH v2] mm/memblock: Correct totalram_pages accounting with KMSAN

[David Hildenbrand <david@redhat.com>]

On 25.09.25 14:37, SeongJae Park wrote:
> Hello,
> 
> On Wed, 24 Sep 2025 12:03:01 +0200 Alexander Potapenko <glider@google.com> wrote:
> 
>> When KMSAN is enabled, `kmsan_memblock_free_pages()` can hold back pages
>> for metadata instead of returning them to the early allocator. The callers,
>> however, would unconditionally increment `totalram_pages`, assuming the
>> pages were always freed. This resulted in an incorrect calculation of the
>> total available RAM, causing the kernel to believe it had more memory than
>> it actually did.
>>
>> This patch refactors `memblock_free_pages()` to return the number of pages
>> it successfully frees. If KMSAN stashes the pages, the function now
>> returns 0; otherwise, it returns the number of pages in the block.
>>
>> The callers in `memblock.c` have been updated to use this return value,
>> ensuring that `totalram_pages` is incremented only by the number of pages
>> actually returned to the allocator. This corrects the total RAM accounting
>> when KMSAN is active.
>>
>> Cc: Aleksandr Nogikh <nogikh@google.com>
>> Fixes: 3c2065098260 ("init: kmsan: call KMSAN initialization routines")
>> Signed-off-by: Alexander Potapenko <glider@google.com>
>> Reviewed-by: David Hildenbrand <david@redhat.com>
> [...]
>> --- a/mm/mm_init.c
>> +++ b/mm/mm_init.c
>> @@ -2548,24 +2548,25 @@ void *__init alloc_large_system_hash(const char *tablename,
>>   	return table;
>>   }
>>   
>> -void __init memblock_free_pages(struct page *page, unsigned long pfn,
>> -							unsigned int order)
>> +unsigned long __init memblock_free_pages(struct page *page, unsigned long pfn,
>> +					 unsigned int order)
>>   {
>>   	if (IS_ENABLED(CONFIG_DEFERRED_STRUCT_PAGE_INIT)) {
>>   		int nid = early_pfn_to_nid(pfn);
>>   
>>   		if (!early_page_initialised(pfn, nid))
>> -			return;
>> +			return 0;
>>   	}
> 
> I found this patch on mm-new tree is making my test machine (QEMU) reports much
> less MemTotal even though KMSAN is disabled.  And modifying the above part to
> be considered as free success (returning '1UL << order') fixed my issue.
> Because the commit message says the purpose of this change is only for
> KMSAN-stashed memory, maybe the above behavior change is not really intended?
> 
> I'm not familiar with this code so I'm unsure if the workaround is the right
> fix.  But since I have no time to look this in deep for now, reporting first.

Good point, I think there is something off here.

-- 
Cheers

David / dhildenb