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

[Ge Yang <yangge1116@126.com>]

在 2025/2/18 16:55, David Hildenbrand 写道:
> On 15.02.25 06:50, Ge Yang wrote:
>>
>>
>> 在 2025/2/14 16:08, David Hildenbrand 写道:
>>> 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.
>>>
>> It seems that we only track unmovable, reclaimable, and movable pages on
>> the pcp lists. For specific details, please refer to the
>> free_frozen_pages() function.
> 
> It reminded me about PCP caches, because we effectively also have to 
> wait for some stuck folios to properly get freed to the buddy.
> 
It seems that when an isolated page is freed, it won't be placed back 
into the PCP caches.