From: David Hildenbrand <david@redhat.com>
To: yangge1116@126.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: Fri, 14 Feb 2025 09:08:18 +0100 [thread overview]
Message-ID: <37363b17-88b0-4ccc-a115-8c9f1d83a1b5@redhat.com> (raw)
In-Reply-To: <1739514729-21265-1-git-send-email-yangge1116@126.com>
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.
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?
Thanks for all your effort around making CMA allocations / migration
more reliable.
--
Cheers,
David / dhildenb
next prev parent reply other threads:[~2025-02-14 8:08 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 [this message]
2025-02-15 5:50 ` Ge Yang
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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