Re: [PATCH V4] mm/hugetlb: wait for hugetlb folios to be freed

[Muchun Song <muchun.song@linux.dev>]

> On Feb 19, 2025, at 11:46, yangge1116@126.com wrote:
> 
> From: Ge Yang <yangge1116@126.com>
> 
> Since the introduction of commit c77c0a8ac4c52 ("mm/hugetlb: defer freeing
> of huge pages if in non-task context"), 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 hugetlb folios, these in-use hugetlb folios need to be migrated
> to another location. When there are no available hugetlb folios in the
> free hugetlb pool during the migration of in-use hugetlb folios, new folios
> are allocated from the buddy system. A temporary state is set on the newly
> allocated folio. Upon completion of the hugetlb 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 hugetlb folios
>        ->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_freed_hugetlb_folios(). This function ensures that the hugetlb
> folios are properly released back to the buddy system after their migration
> is completed. By invoking wait_for_freed_hugetlb_folios() before calling
> PageBuddy(), we ensure that PageBuddy() will succeed.
> 
> Fixes: c77c0a8ac4c52 ("mm/hugetlb: defer freeing of huge pages if in non-task context")
> Signed-off-by: Ge Yang <yangge1116@126.com>
> Cc: <stable@vger.kernel.org>

Reviewed-by: Muchun Song <muchun.song@linux.dev>

Thanks.