Re: [PATCH V2] mm/hugetlb: wait for hugepage folios to be freed

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 15 Feb 2025 15:20:26 +0800 yangge1116@126.com wrote:

> From: Ge Yang <yangge1116@126.com>
> 
> Since the introduction of commit b65d4adbc0f0 ("mm: hugetlb: defer freeing
> of HugeTLB pages"), which supports deferring the freeing of HugeTLB pages,
> the allocation of contiguous memory through cma_alloc() may fail
> probabilistically.
> 
> In the CMA allocation process, if it is found that the CMA area is occupied
> by in-use hugepage folios, these in-use hugepage folios need to be migrated
> to another location. When there are no available hugepage folios in the
> free HugeTLB pool during the migration of in-use HugeTLB pages, new folios
> are allocated from the buddy system. A temporary state is set on the newly
> allocated folio. Upon completion of the hugepage folio migration, the
> temporary state is transferred from the new folios to the old folios.
> Normally, when the old folios with the temporary state are freed, it is
> directly released back to the buddy system. However, due to the deferred
> freeing of HugeTLB pages, the PageBuddy() check fails, ultimately leading
> to the failure of cma_alloc().
> 
> Here is a simplified call trace illustrating the process:
> cma_alloc()
>     ->__alloc_contig_migrate_range() // Migrate in-use hugepage
>         ->unmap_and_move_huge_page()
>             ->folio_putback_hugetlb() // Free old folios
>     ->test_pages_isolated()
>         ->__test_page_isolated_in_pageblock()
>              ->PageBuddy(page) // Check if the page is in buddy
> 
> To resolve this issue, we have implemented a function named
> wait_for_hugepage_folios_freed(). This function ensures that the hugepage
> folios are properly released back to the buddy system after their migration
> is completed. By invoking wait_for_hugepage_folios_freed() before calling
> PageBuddy(), we ensure that PageBuddy() will succeed.
> 
> Fixes: b65d4adbc0f0 ("mm: hugetlb: defer freeing of HugeTLB pages")

Do you feel that this issue is serious enough to justify a -stable
backport of the fix?