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[91.12.97.162]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38f259f8273sm14184140f8f.89.2025.02.18.01.41.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 18 Feb 2025 01:41:57 -0800 (PST) Message-ID: <048ca765-bf44-46ab-87d4-328dc0979159@redhat.com> Date: Tue, 18 Feb 2025 10:41:56 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/hugetlb: wait for hugepage folios to be freed To: Ge Yang , 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 References: <1739514729-21265-1-git-send-email-yangge1116@126.com> <37363b17-88b0-4ccc-a115-8c9f1d83a1b5@redhat.com> <2d0b01c5-a736-41d5-a0f7-db0da065d049@redhat.com> <406c6713-356b-4acf-bcd0-e5a6c1e9adcf@126.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 18.02.25 10:22, Ge Yang wrote: > > > 在 2025/2/18 16:55, David Hildenbrand 写道: >> On 15.02.25 06:50, Ge Yang wrote: >>> >>> >>> 在 2025/2/14 16:08, David Hildenbrand 写道: >>>> On 14.02.25 07:32, yangge1116@126.com wrote: >>>>> From: Ge Yang >>>>> >>>>> 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. >> >> It reminded me about PCP caches, because we effectively also have to >> wait for some stuck folios to properly get freed to the buddy. >> > It seems that when an isolated page is freed, it won't be placed back > into the PCP caches. I recall there are cases when the page was in the pcp before the isolation started, which is why we drain the pcp at some point (IIRC). -- Cheers, David / dhildenb