From: Ge Yang <yangge1116@126.com>
To: David Hildenbrand <david@redhat.com>, akpm@linux-foundation.org
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
stable@vger.kernel.org, 21cnbao@gmail.com,
baolin.wang@linux.alibaba.com, muchun.song@linux.dev,
osalvador@suse.de, liuzixing@hygon.cn
Subject: Re: [PATCH] mm/hugetlb: wait for hugepage folios to be freed
Date: Sat, 15 Feb 2025 13:50:15 +0800 [thread overview]
Message-ID: <d043bdd2-a978-4a09-869e-b6e43f5ce409@126.com> (raw)
In-Reply-To: <37363b17-88b0-4ccc-a115-8c9f1d83a1b5@redhat.com>
在 2025/2/14 16:08, David Hildenbrand 写道:
> On 14.02.25 07:32, yangge1116@126.com wrote:
>> From: Ge Yang <yangge1116@126.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.
> I wonder if that waiting should instead be done exactly once after
> migrating multiple folios? For example, at the beginning of
> test_pages_isolated(), to "flush" that state from any previous migration?
>
Yes, this can improve performance. I will make the modification in the
next version. Thank you.
> Thanks for all your effort around making CMA allocations / migration
> more reliable.
>
next prev parent reply other threads:[~2025-02-15 5:50 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-02-14 6:32 [PATCH] mm/hugetlb: wait for hugepage folios to be freed yangge1116
2025-02-14 6:33 ` kernel test robot
2025-02-14 8:08 ` David Hildenbrand
2025-02-15 5:50 ` Ge Yang [this message]
2025-02-18 8:55 ` David Hildenbrand
2025-02-18 9:22 ` Ge Yang
2025-02-18 9:41 ` David Hildenbrand
2025-02-18 9:54 ` Ge Yang
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