Re: [PATCH] mm/page_alloc: find_large_buddy() from start_pfn aligned order

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

On Thu, Aug 28, 2025 at 11:02:33PM -0400, Zi Yan wrote:
[...]
>
>BTW, I also notice that when start_pfn is an order-0 PageBuddy, the
>"if (pfn + (1 << buddy_order(page)) > start_pfn)" check below would be true
>even if there is no buddy straddles start_pfn, although "return pfn"
>gives the same results as "return start_pfn" (no straddle). The original
>code before the addition of find_large_buddy() (commit fd919a85cd55 ("mm:
>page_isolation: prepare for hygienic freelists")) checks start_pfn == pfn
>before the straddle check, so the correct code should check start_pfn == pfn
>and return early. But since current code is functionally equivalent.
>Maybe adding a comment about it would be sufficient. Something like:
>
>When the found buddy order is 0, the check would give false positive,
>but the returned result is still correct, since pfn is the same as start_pfn.

Hm... found another thing.

In __move_freepages_block_isolate(), when checking "We're the starting block
of a large buddy", variable "page" may not be the large buddy page we found
from find_large_buddy(). Since prep_move_freepages_block() set start_pfn to
pageblock_start_pfn(page_to_pfn(page)), which may be different. 

-- 
Wei Yang
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