Message-ID: <4195424c-3147-402f-bb73-4f7b96f6a9bf@redhat.com>
Date: Tue, 18 Feb 2025 09:52:35 +0100
From: David Hildenbrand <david@...hat.com>
To: Muchun Song <muchun.song@...ux.dev>, yangge1116@....com
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, 21cnbao@...il.com,
 baolin.wang@...ux.alibaba.com, osalvador@...e.de, liuzixing@...on.cn
Subject: Re: [PATCH V2] mm/hugetlb: wait for hugepage folios to be freed

On 18.02.25 07:52, Muchun Song wrote:
> 
> 
>> On Feb 15, 2025, at 15:20, 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")
>> Signed-off-by: Ge Yang <yangge1116@....com>
>> ---
>>
>> V2:
>> - flush all folios at once suggested by David
>>
>> include/linux/hugetlb.h |  5 +++++
>> mm/hugetlb.c            |  8 ++++++++
>> mm/page_isolation.c     | 10 ++++++++++
>> 3 files changed, 23 insertions(+)
>>
>> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
>> index 6c6546b..04708b0 100644
>> --- a/include/linux/hugetlb.h
>> +++ b/include/linux/hugetlb.h
>> @@ -697,6 +697,7 @@ bool hugetlb_bootmem_page_zones_valid(int nid, 
>> struct huge_bootmem_page *m);
>>
>> int isolate_or_dissolve_huge_page(struct page *page, struct list_head 
>> *list);
>> int replace_free_hugepage_folios(unsigned long start_pfn, unsigned 
>> long end_pfn);
>> +void wait_for_hugepage_folios_freed(void);
>> struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
>>                unsigned long addr, bool cow_from_owner);
>> struct folio *alloc_hugetlb_folio_nodemask(struct hstate *h, int 
>> preferred_nid,
>> @@ -1092,6 +1093,10 @@ static inline int 
>> replace_free_hugepage_folios(unsigned long start_pfn,
>>    return 0;
>> }
>>
>> +static inline void wait_for_hugepage_folios_freed(void)
>> +{
>> +}
>> +
>> static inline struct folio *alloc_hugetlb_folio(struct vm_area_struct 
>> *vma,
>>                       unsigned long addr,
>>                       bool cow_from_owner)
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 30bc34d..36dd3e4 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -2955,6 +2955,14 @@ int replace_free_hugepage_folios(unsigned long 
>> start_pfn, unsigned long end_pfn)
>>    return ret;
>> }
>>
>> +void wait_for_hugepage_folios_freed(void)
> 
> We usually use the "hugetlb" term now instead of "huge_page" to 
> differentiate with THP. So I suggest naming it as 
> wait_for_hugetlb_folios_freed().

Maybe "wait_for_freed_hugetlb_folios" or "hugetlb_wait_for_freed_folios".

In general, LGTM

-- 
Cheers,

David / dhildenb

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