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

[Zi Yan <ziy@nvidia.com>]

On 23 Sep 2025, at 3:48, David Hildenbrand wrote:

> 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 ...

Something like the code below:

pfn = ALIGN(zone->zone_start_pfn, rounddown_pow_of_two(nr_pages));

Best Regards,
Yan, Zi