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

[Wei Yang <richard.weiyang@gmail.com>]

On Tue, Sep 23, 2025 at 09:29:12AM +0200, Michal Hocko wrote:
>On Tue 23-09-25 00:19:43, 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.
>> 
>> Use roundup() to calculate the correct alignment.
>
>What is the problem you are trying to fix here? The ALIGN is certainly
>not well defined for nr_pages that is not power of two but does any
>caller of alloc_contig_pages assumes any specific alignment, especially
>when nr_pages is not power of two?
>

There is no real problem here. The initial thought is ALIGN() don't expect
non-power_of_two value.

The discussion above sounds reasonable, we don't need alignment for
non-power_of_two nr_pages.

So this looks we can start iteration from zone->zone_start_pfn for
non-power_of_two nr_pages.

IMHO, How about the change below?

    pfn = zone->zone_start_pfn;
    if (is_power_of_2(nr_pages))
        pfn = ALIGN(pfn, nr_pages)

And it looks making code consist with the doc above.

-- 
Wei Yang
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