Re: [PATCH] mm/page_alloc: fix alignment for alloc_contig_pages_noprof()

[David Hildenbrand <david@redhat.com>]

On 23.09.25 09:06, Anshuman Khandual wrote:
> On 23/09/25 7:16 AM, Zi Yan wrote:
>> On 22 Sep 2025, at 20:19, Wei Yang wrote:
>>
>>> Commit 5e27a2df03b8 ("mm/page_alloc: add alloc_contig_pages()")
>>> introduced generic method for alloc_contig_pages(). But the alignment
>>> calculation seems wrong.
>>>
>>> Since ALIGN() only accept power of two value, while nr_pages could be
>>> any positive one, the result is not defined.
>>
>> The result would not be any value lower than zone->zone_start_pfn,
>> so the worst case is getting an unaligned PFN range. I guess
>> most of the time nr_pages would be power of 2.
> 
> Agreed.
> 
> Also as Dev had pointed out earlier, this function gives no
> guarantee on alignment of non-power-of-2 requests. Hence I
> don't have a strong opinion either way, but does it really
> qualify for a "Fixes:" tag ?

I'd say if there is nothing to fix, then this patch is not required.

What likely does make sense would be that a non-power-of-2 would be 
aligned to the smallest contained power of 2.

E.g., a 6 MiB request would be aligned to 2 MiB instead of multiples of 
6 MiB.

Not sure what the existing ALIGN would do with that ...

-- 
Cheers

David / dhildenb