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

[Ge Yang <yangge1116@126.com>]

在 2025/2/18 17:41, David Hildenbrand 写道:
> On 18.02.25 10:22, Ge Yang wrote:
>>
>>
>> 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.
> 
> I recall there are cases when the page was in the pcp before the 
> isolation started, which is why we drain the pcp at some point (IIRC).
> 
Yes, indeed, drain_all_pages(cc.zone) is currently executed before 
__alloc_contig_migrate_range().