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[2003:cb:c726:6800:7ddf:5fc:2ee5:f08a]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-39eae979663sm1359590f8f.51.2025.04.11.01.42.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 11 Apr 2025 01:42:10 -0700 (PDT) Message-ID: <6b532646-041f-4077-b09f-ff6d43aa4a81@redhat.com> Date: Fri, 11 Apr 2025 10:42:09 +0200 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: [RFC PATCH] mm: huge_memory: add folio_mark_accessed() when zapping file THP To: Baolin Wang , Barry Song <21cnbao@gmail.com>, Zi Yan Cc: akpm@linux-foundation.org, willy@infradead.org, ryan.roberts@arm.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <34bab7a60930472377afbfeefe05b980d0512aa4.1744118089.git.baolin.wang@linux.alibaba.com> <5c52b67a-8e7e-4dd7-9127-96944715d883@linux.alibaba.com> <1E123113-7A0B-4D3A-AC7A-01767D7BF2D8@nvidia.com> <247fca57-8280-41a6-85b0-03a32ea08210@linux.alibaba.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 11.04.25 03:20, Baolin Wang wrote: > > > On 2025/4/11 05:56, Barry Song wrote: >> On Fri, Apr 11, 2025 at 3:13 AM Zi Yan wrote: >>> >>> On 10 Apr 2025, at 6:29, Barry Song wrote: >>> >>>> On Thu, Apr 10, 2025 at 9:05 PM Baolin Wang >>>> wrote: >>>>> >>>>> >>>>> >>>>> On 2025/4/10 16:14, Barry Song wrote: >>>>>> On Wed, Apr 9, 2025 at 1:16 AM Baolin Wang >>>>>> wrote: >>>>>>> >>>>>>> When investigating performance issues during file folio unmap, I noticed some >>>>>>> behavioral differences in handling non-PMD-sized folios and PMD-sized folios. >>>>>>> For non-PMD-sized file folios, it will call folio_mark_accessed() to mark the >>>>>>> folio as having seen activity, but this is not done for PMD-sized folios. >>>>>>> >>>>>>> This might not cause obvious issues, but a potential problem could be that, >>>>>>> it might lead to more frequent refaults of PMD-sized file folios under memory >>>>>>> pressure. Therefore, I am unsure whether the folio_mark_accessed() should be >>>>>>> added for PMD-sized file folios? >>>>>>> >>>>>>> Signed-off-by: Baolin Wang >>>>>>> --- >>>>>>> mm/huge_memory.c | 4 ++++ >>>>>>> 1 file changed, 4 insertions(+) >>>>>>> >>>>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>>>>>> index 6ac6d468af0d..b3ade7ac5bbf 100644 >>>>>>> --- a/mm/huge_memory.c >>>>>>> +++ b/mm/huge_memory.c >>>>>>> @@ -2262,6 +2262,10 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma, >>>>>>> zap_deposited_table(tlb->mm, pmd); >>>>>>> add_mm_counter(tlb->mm, mm_counter_file(folio), >>>>>>> -HPAGE_PMD_NR); >>>>>>> + >>>>>>> + if (flush_needed && pmd_young(orig_pmd) && >>>>>>> + likely(vma_has_recency(vma))) >>>>>>> + folio_mark_accessed(folio); >>>>>> >>>>>> Acked-by: Barry Song >>>>> >>>>> Thanks. >>>>> >>>>>> I also came across an interesting observation: on a memory-limited system, >>>>>> demoting unmapped file folios in the LRU—specifically when their mapcount >>>>>> drops from 1 to 0—can actually improve performance. >>>>> >>>>> These file folios are used only once? Can folio_set_dropbehind() be used >>>>> to optimize it, which can avoid the LRU activity movement in >>>>> folio_mark_accessed()? >>>> >>>> For instance, when a process, such as a game, just exits, it can be expected >>>> that it won't be used again in the near future. As a result, demoting >>>> its previously >>>> unmapped file pages can improve performance. >>> >>> Is it possible to mark the dying VMAs either VM_SEQ_READ or VM_RAND_READ >>> so that folio_mark_accessed() will be skipped? Or a new vm_flag? >>> Will it work? >> >> Actually took a more aggressive approach and observed good performance >> improvements on phones. After zap_pte_range() called remove_rmap(), >> the following logic was added: >> >> if (file_folio && !folio_mapped()) >> deactivate_file_folio(); >> >> This helps file folios from exiting processes get reclaimed more quickly >> during the MGLRU's min generation scan while the folios are probably >> in max gen. >> >> I'm not entirely sure if this is universally applicable or worth submitting as >> a patch. > > IMHO, I'm afraid this is not universally applicable. Although these file > folios have been unmapped, it's not certain that they won't be accessed > again. These file folios might be remapped and accessed again soon, or > accessed through read()/write() operations using a file descriptor. > > I agree with Zi's suggestion. Using some kind of madvise() hint to mark > these file folios as those that won't be accessed after being unmapped, > seems can work? Is that similar to MADV_COLD before unmap? -- Cheers, David / dhildenb