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

[David Hildenbrand <david@redhat.com>]

On 14.02.25 07:32, 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() following the
> migration process, we guarantee that when test_pages_isolated() is
> executed, it will successfully pass.

Okay, so after every successful migration -> put of src, we wait for the 
src to actually get freed.

When migrating multiple hugetlb folios, we'd wait once per folio.

It reminds me a bit about pcp caches, where folios are !buddy until the 
pcp was drained.

I wonder if that waiting should instead be done exactly once after 
migrating multiple folios? For example, at the beginning of 
test_pages_isolated(), to "flush" that state from any previous migration?

Thanks for all your effort around making CMA allocations / migration 
more reliable.

-- 
Cheers,

David / dhildenb