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

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Message-ID: <462e8d90-c0d0-474b-851c-46a44282b768@126.com>
Date: Wed, 19 Feb 2025 11:40:02 +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 2025/2/19 1:22, David Hildenbrand 写道:
> On 18.02.25 13:19, Ge Yang wrote:
>>
>>
>> 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?
> 
> I think that's better, except that it would still trigger in scenarios 
> where hugetlb is completely unused if 
> CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP is around.
> 
> Can't we check hpage_freelist?
> 
> if (llist_empty(&hpage_freelist))
>      return;
> flush_work(&free_hpage_work);
> 
Ok, thanks.
> It should be able to deal with races (we don't care if something is 
> getting added concurrently, only if there is something right now).
>