linux-kernel - Re: [PATCH V3] mm/hugetlb: wait for hugetlb folios to be freed

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Message-ID: <17ad5bf5-545c-4418-8d08-459ce6ef54cb@126.com>
Date: Tue, 18 Feb 2025 20:19:58 +0800
From: Ge Yang <yangge1116@....com>
To: David Hildenbrand <david@...hat.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 V3] mm/hugetlb: wait for hugetlb folios to be freed



在 2025/2/18 19:45, David Hildenbrand 写道:
> On 18.02.25 12:40, yangge1116@....com wrote:
>> From: Ge Yang <yangge1116@....com>
>>
>> Since the introduction of commit c77c0a8ac4c52 ("mm/hugetlb: defer 
>> freeing
>> of huge pages if in non-task context"), 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 hugetlb folios, these in-use hugetlb folios need to be migrated
>> to another location. When there are no available hugetlb folios in the
>> free hugetlb pool during the migration of in-use hugetlb folios, new 
>> folios
>> are allocated from the buddy system. A temporary state is set on the 
>> newly
>> allocated folio. Upon completion of the hugetlb 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 hugetlb folios
>>          ->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_freed_hugetlb_folios(). This function ensures that the hugetlb
>> folios are properly released back to the buddy system after their 
>> migration
>> is completed. By invoking wait_for_freed_hugetlb_folios() before calling
>> PageBuddy(), we ensure that PageBuddy() will succeed.
>>
>> Fixes: c77c0a8ac4c52 ("mm/hugetlb: defer freeing of huge pages if in 
>> non-task context")
>> Signed-off-by: Ge Yang <yangge1116@....com>
>> Cc: <stable@...r.kernel.org>
> 
> 
> 
> Acked-by: David Hildenbrand <david@...hat.com>
>> +void wait_for_freed_hugetlb_folios(void)
>> +{
>> +    flush_work(&free_hpage_work);
> 
> BTW, I was wondering if we could optimize out some calls here by sensing 
> if there is actually work.
> 
for_each_hstate(h) {
	if (hugetlb_vmemmap_optimizable(h)) {
		flush_work(&free_hpage_work);
		break;
	}
}
Is this adjustment okay?
> In most cases, we'll never ever have to actually wait here, especially 
> on systems without any configured hugetlb pages etc ...
> 
> It's rather a corner case that we have to wait here on most systems.
>