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

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Message-ID: <2d0b01c5-a736-41d5-a0f7-db0da065d049@redhat.com>
Date: Tue, 18 Feb 2025 09:55:00 +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 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.

-- 
Cheers,

David / dhildenb