linux-kernel - Re: [PATCH V2] mm/hugetlb: wait for hugepage folios to be freed

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Message-ID: <1ea2c7b0-790c-4f60-a713-9029252a0837@126.com>
Date: Tue, 18 Feb 2025 15:24:01 +0800
From: Ge Yang <yangge1116@....com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, 21cnbao@...il.com,
 david@...hat.com, baolin.wang@...ux.alibaba.com, muchun.song@...ux.dev,
 osalvador@...e.de, liuzixing@...on.cn
Subject: Re: [PATCH V2] mm/hugetlb: wait for hugepage folios to be freed



在 2025/2/18 12:34, Andrew Morton 写道:
> On Sat, 15 Feb 2025 15:20:26 +0800 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() before calling
>> PageBuddy(), we ensure that PageBuddy() will succeed.
>>
>> Fixes: b65d4adbc0f0 ("mm: hugetlb: defer freeing of HugeTLB pages")
> 
> Do you feel that this issue is serious enough to justify a -stable
> backport of the fix?
Yes, I will add 'CC: stable@...r.kernel.org' in the next patch, thanks.