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Wed, 07 Jan 2026 10:29:19 +0800 Message-ID: Date: Wed, 7 Jan 2026 10:29:18 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 5/5] mm: rmap: support batched unmapping for file large folios To: Barry Song <21cnbao@gmail.com>, Wei Yang Cc: akpm@linux-foundation.org, david@kernel.org, catalin.marinas@arm.com, will@kernel.org, lorenzo.stoakes@oracle.com, ryan.roberts@arm.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, riel@surriel.com, harry.yoo@oracle.com, jannh@google.com, willy@infradead.org, dev.jain@arm.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <142919ac14d3cf70cba370808d85debe089df7b4.1766631066.git.baolin.wang@linux.alibaba.com> <20260106132203.kdxfvootlkxzex2l@master> <20260107014601.dxvq6b7ljgxwg7iu@master> From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 604E240007 X-Stat-Signature: qi8o3t7frj15gffa89qss373u4mzfbs3 X-Rspam-User: X-HE-Tag: 1767752965-239155 X-HE-Meta: U2FsdGVkX19WDKYDL4x4v/59+qXGNhRrl/iHbwaVSml2rbMjg6k9g5QblITdCFvPvteY7KWraE8Nwsg/PsAcvfcpZUOREKnEI9XHxjQf1oaJWOpeoPgBy640J9fT/h0diwMAHVRPg5w1NvbufbJ+11FrbfWlMvMVt2HhfCF9BEZkCd4Q4F1CdrVdBVuMhsoYt7UPdwtJGjwYkefjL46Qf80OU1Gg//5HMjoPpO8sojpCvkzi7iliohGVS7BesHvPS25K0uBCn9uIKUxFaunEHZ52kUQ5etL6b4TwmBowMVSqX132qUs2njjAYDAc+U2gQ4FuYCaIgEhceqBP1R9RzlumUxNk+HihyQ5zdIGaAXmPfYonK0o/2JR429nJZTrfjRu1gx26e6/Gm/XuujrGgHYUwqoiibXe7CNgVX7oZC29EaHMrMvOgOXpc8036VzfbBFg1Lyw8nJ1FWZGvxV8FS4NDywOegbc+Ud4yeHpG5yjzNWotp6IzbR0W4B/j4G07iWTbKCXZ9YPJS+cJ/2CyGCfuSzV+Kue5TvKjPahg1xb3IJBbfQTdRUOr4QjujIZ7JIEnrL/fmqgx33quTYRLJ5pu+uXsSV/JhvgbvOukDAhSoEDvv3oHPJiJ6sU2Ft+e5F1CfkFumL8Dpe7sDyoaPyZ7ZHvnr4hReA6fjd/IMi2NNXxksSsiOAx5b805T7EfDytPZtvcHh1N7Qaa0Fp/mIajiXLTHhGKKGJSUhdenj9OcP58NDYYpJu+bmPmDK5/frUOEApxPmUX45Vad2ERfZtaB0lUAnUiudLZ4H4Fzfy+px/YNDlH7pO47myIM7+4JhgrOKUCbxuDogtrh87FPIKRvB9atzJtU0BeMk0n4kpNBsgDMe9iRPlgV2VoZdMfztv32HpPSpG4m4Yx/fXz9AxEH2KyWBdKM9kPig8olw0qdpO94LvNIfN1FIwu1yOQTs3sNlqAkvAV7T8o4v S9KXb3et 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 1/7/26 10:21 AM, Barry Song wrote: > On Wed, Jan 7, 2026 at 2:46 PM Wei Yang wrote: >> >> On Wed, Jan 07, 2026 at 10:29:25AM +1300, Barry Song wrote: >>> On Wed, Jan 7, 2026 at 2:22 AM Wei Yang wrote: >>>> >>>> On Fri, Dec 26, 2025 at 02:07:59PM +0800, Baolin Wang wrote: >>>>> Similar to folio_referenced_one(), we can apply batched unmapping for file >>>>> large folios to optimize the performance of file folios reclamation. >>>>> >>>>> Barry previously implemented batched unmapping for lazyfree anonymous large >>>>> folios[1] and did not further optimize anonymous large folios or file-backed >>>>> large folios at that stage. As for file-backed large folios, the batched >>>>> unmapping support is relatively straightforward, as we only need to clear >>>>> the consecutive (present) PTE entries for file-backed large folios. >>>>> >>>>> Performance testing: >>>>> Allocate 10G clean file-backed folios by mmap() in a memory cgroup, and try to >>>>> reclaim 8G file-backed folios via the memory.reclaim interface. I can observe >>>>> 75% performance improvement on my Arm64 32-core server (and 50%+ improvement >>>>> on my X86 machine) with this patch. >>>>> >>>>> W/o patch: >>>>> real 0m1.018s >>>>> user 0m0.000s >>>>> sys 0m1.018s >>>>> >>>>> W/ patch: >>>>> real 0m0.249s >>>>> user 0m0.000s >>>>> sys 0m0.249s >>>>> >>>>> [1] https://lore.kernel.org/all/20250214093015.51024-4-21cnbao@gmail.com/T/#u >>>>> Reviewed-by: Ryan Roberts >>>>> Acked-by: Barry Song >>>>> Signed-off-by: Baolin Wang >>>>> --- >>>>> mm/rmap.c | 7 ++++--- >>>>> 1 file changed, 4 insertions(+), 3 deletions(-) >>>>> >>>>> diff --git a/mm/rmap.c b/mm/rmap.c >>>>> index 985ab0b085ba..e1d16003c514 100644 >>>>> --- a/mm/rmap.c >>>>> +++ b/mm/rmap.c >>>>> @@ -1863,9 +1863,10 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio, >>>>> end_addr = pmd_addr_end(addr, vma->vm_end); >>>>> max_nr = (end_addr - addr) >> PAGE_SHIFT; >>>>> >>>>> - /* We only support lazyfree batching for now ... */ >>>>> - if (!folio_test_anon(folio) || folio_test_swapbacked(folio)) >>>>> + /* We only support lazyfree or file folios batching for now ... */ >>>>> + if (folio_test_anon(folio) && folio_test_swapbacked(folio)) >>>>> return 1; >>>>> + >>>>> if (pte_unused(pte)) >>>>> return 1; >>>>> >>>>> @@ -2231,7 +2232,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, >>>>> * >>>>> * See Documentation/mm/mmu_notifier.rst >>>>> */ >>>>> - dec_mm_counter(mm, mm_counter_file(folio)); >>>>> + add_mm_counter(mm, mm_counter_file(folio), -nr_pages); >>>>> } >>>>> discard: >>>>> if (unlikely(folio_test_hugetlb(folio))) { >>>>> -- >>>>> 2.47.3 >>>>> >>>> >>>> Hi, Baolin >>>> >>>> When reading your patch, I come up one small question. >>>> >>>> Current try_to_unmap_one() has following structure: >>>> >>>> try_to_unmap_one() >>>> while (page_vma_mapped_walk(&pvmw)) { >>>> nr_pages = folio_unmap_pte_batch() >>>> >>>> if (nr_pages = folio_nr_pages(folio)) >>>> goto walk_done; >>>> } >>>> >>>> I am thinking what if nr_pages > 1 but nr_pages != folio_nr_pages(). >>>> >>>> If my understanding is correct, page_vma_mapped_walk() would start from >>>> (pvmw->address + PAGE_SIZE) in next iteration, but we have already cleared to >>>> (pvmw->address + nr_pages * PAGE_SIZE), right? >>>> >>>> Not sure my understanding is correct, if so do we have some reason not to >>>> skip the cleared range? >>> >>> I don’t quite understand your question. For nr_pages > 1 but not equal >>> to nr_pages, page_vma_mapped_walk will skip the nr_pages - 1 PTEs inside. >>> >>> take a look: >>> >>> next_pte: >>> do { >>> pvmw->address += PAGE_SIZE; >>> if (pvmw->address >= end) >>> return not_found(pvmw); >>> /* Did we cross page table boundary? */ >>> if ((pvmw->address & (PMD_SIZE - PAGE_SIZE)) == 0) { >>> if (pvmw->ptl) { >>> spin_unlock(pvmw->ptl); >>> pvmw->ptl = NULL; >>> } >>> pte_unmap(pvmw->pte); >>> pvmw->pte = NULL; >>> pvmw->flags |= PVMW_PGTABLE_CROSSED; >>> goto restart; >>> } >>> pvmw->pte++; >>> } while (pte_none(ptep_get(pvmw->pte))); >>> >> >> Yes, we do it in page_vma_mapped_walk() now. Since they are pte_none(), they >> will be skipped. >> >> I mean maybe we can skip it in try_to_unmap_one(), for example: >> >> diff --git a/mm/rmap.c b/mm/rmap.c >> index 9e5bd4834481..ea1afec7c802 100644 >> --- a/mm/rmap.c >> +++ b/mm/rmap.c >> @@ -2250,6 +2250,10 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, >> */ >> if (nr_pages == folio_nr_pages(folio)) >> goto walk_done; >> + else { >> + pvmw.address += PAGE_SIZE * (nr_pages - 1); >> + pvmw.pte += nr_pages - 1; >> + } >> continue; >> walk_abort: >> ret = false; > > > I feel this couples the PTE walk iteration with the unmap > operation, which does not seem fine to me. It also appears > to affect only corner cases. Agree. There may be no performance gains, so I also prefer to leave it as is.