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

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Message-ID: <048ca765-bf44-46ab-87d4-328dc0979159@redhat.com>
Date: Tue, 18 Feb 2025 10:41:56 +0100
From: David Hildenbrand <david@...hat.com>
To: Ge Yang <yangge1116@....com>, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org,
 21cnbao@...il.com, baolin.wang@...ux.alibaba.com, muchun.song@...ux.dev,
 osalvador@...e.de, liuzixing@...on.cn
Subject: Re: [PATCH] mm/hugetlb: wait for hugepage folios to be freed

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@....com wrote:
>>>>> From: Ge Yang <yangge1116@....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).

-- 
Cheers,

David / dhildenb