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Tue, 10 Mar 2026 09:37:56 +0800 Message-ID: Date: Tue, 10 Mar 2026 09:37:55 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 1/5] mm: rmap: support batched checks of the references for large folios To: Barry Song <21cnbao@gmail.com> 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: <12132694536834262062d1fb304f8f8a064b6750.1770645603.git.baolin.wang@linux.alibaba.com> From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Stat-Signature: ycgf8tmysaw81ga8kh8gk77bcw7icif6 X-Rspam-User: X-Rspamd-Queue-Id: 2E47CA0007 X-Rspamd-Server: rspam12 X-HE-Tag: 1773106681-42814 X-HE-Meta: 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 8ewTx2de ZcJuRQACWB+9PwFS6wfyulWai1EcoXC0umJVNzloFwcgAouYHXGWtPZpc1KG65NL9qY9J3nsJl9W+I3Go42E90Oemv7wnr9S1RIAfPhIGIcOBcD85CGR9O2dNoxVHM3iM6JPutqXVACEJ5V48ujwcUMEaD6/zFM50qbReTwiB5hMnFcyeIQ5EVFFIrfdP3DtXsX5LJyk6M/IwVeo5iS+/EwqftpXI4aJq4xSxtGewQx+X3YnMdBOf9yvHcbsGs6kg92YKVtJGWqCFqNWpbUR4LkETtBlBGHkPMqiE5Zl8aWXgVcLY98KYh5pWWt9oiHDYyTbPf2x1kNB4j9+5VTHrTMcc6g8nYL27iYdgGWRYRRCdN4GYi6rotnKtUXQ23OMxhJDp3z50oeG8DR+TVbrbMOfZg78q269T514u81KzUcqzRD/KQxqMyWfvVMYsrgab5F+I3IgDKWct7G4/4uYqwm1hew== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/7/26 4:02 PM, Barry Song wrote: > On Sat, Mar 7, 2026 at 10:22 AM Baolin Wang > wrote: >> >> >> >> On 3/7/26 5:07 AM, Barry Song wrote: >>> On Mon, Feb 9, 2026 at 10:07 PM Baolin Wang >>> wrote: >>>> >>>> Currently, folio_referenced_one() always checks the young flag for each PTE >>>> sequentially, which is inefficient for large folios. This inefficiency is >>>> especially noticeable when reclaiming clean file-backed large folios, where >>>> folio_referenced() is observed as a significant performance hotspot. >>>> >>>> Moreover, on Arm64 architecture, which supports contiguous PTEs, there is already >>>> an optimization to clear the young flags for PTEs within a contiguous range. >>>> However, this is not sufficient. We can extend this to perform batched operations >>>> for the entire large folio (which might exceed the contiguous range: CONT_PTE_SIZE). >>>> >>>> Introduce a new API: clear_flush_young_ptes() to facilitate batched checking >>>> of the young flags and flushing TLB entries, thereby improving performance >>>> during large folio reclamation. And it will be overridden by the architecture >>>> that implements a more efficient batch operation in the following patches. >>>> >>>> While we are at it, rename ptep_clear_flush_young_notify() to >>>> clear_flush_young_ptes_notify() to indicate that this is a batch operation. >>>> >>>> Reviewed-by: Harry Yoo >>>> Reviewed-by: Ryan Roberts >>>> Signed-off-by: Baolin Wang >>> >>> LGTM, >>> >>> Reviewed-by: Barry Song >> >> Thanks. >> >>>> --- >>>> include/linux/mmu_notifier.h | 9 +++++---- >>>> include/linux/pgtable.h | 35 +++++++++++++++++++++++++++++++++++ >>>> mm/rmap.c | 28 +++++++++++++++++++++++++--- >>>> 3 files changed, 65 insertions(+), 7 deletions(-) >>>> >>>> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h >>>> index d1094c2d5fb6..07a2bbaf86e9 100644 >>>> --- a/include/linux/mmu_notifier.h >>>> +++ b/include/linux/mmu_notifier.h >>>> @@ -515,16 +515,17 @@ static inline void mmu_notifier_range_init_owner( >>>> range->owner = owner; >>>> } >>>> >>>> -#define ptep_clear_flush_young_notify(__vma, __address, __ptep) \ >>>> +#define clear_flush_young_ptes_notify(__vma, __address, __ptep, __nr) \ >>>> ({ \ >>>> int __young; \ >>>> struct vm_area_struct *___vma = __vma; \ >>>> unsigned long ___address = __address; \ >>>> - __young = ptep_clear_flush_young(___vma, ___address, __ptep); \ >>>> + unsigned int ___nr = __nr; \ >>>> + __young = clear_flush_young_ptes(___vma, ___address, __ptep, ___nr); \ >>>> __young |= mmu_notifier_clear_flush_young(___vma->vm_mm, \ >>>> ___address, \ >>>> ___address + \ >>>> - PAGE_SIZE); \ >>>> + ___nr * PAGE_SIZE); \ >>>> __young; \ >>>> }) >>>> >>>> @@ -650,7 +651,7 @@ static inline void mmu_notifier_subscriptions_destroy(struct mm_struct *mm) >>>> >>>> #define mmu_notifier_range_update_to_read_only(r) false >>>> >>>> -#define ptep_clear_flush_young_notify ptep_clear_flush_young >>>> +#define clear_flush_young_ptes_notify clear_flush_young_ptes >>>> #define pmdp_clear_flush_young_notify pmdp_clear_flush_young >>>> #define ptep_clear_young_notify ptep_test_and_clear_young >>>> #define pmdp_clear_young_notify pmdp_test_and_clear_young >>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h >>>> index 21b67d937555..a50df42a893f 100644 >>>> --- a/include/linux/pgtable.h >>>> +++ b/include/linux/pgtable.h >>>> @@ -1068,6 +1068,41 @@ static inline void wrprotect_ptes(struct mm_struct *mm, unsigned long addr, >>>> } >>>> #endif >>>> >>>> +#ifndef clear_flush_young_ptes >>>> +/** >>>> + * clear_flush_young_ptes - Mark PTEs that map consecutive pages of the same >>>> + * folio as old and flush the TLB. >>>> + * @vma: The virtual memory area the pages are mapped into. >>>> + * @addr: Address the first page is mapped at. >>>> + * @ptep: Page table pointer for the first entry. >>>> + * @nr: Number of entries to clear access bit. >>>> + * >>>> + * May be overridden by the architecture; otherwise, implemented as a simple >>>> + * loop over ptep_clear_flush_young(). >>>> + * >>>> + * Note that PTE bits in the PTE range besides the PFN can differ. For example, >>>> + * some PTEs might be write-protected. >>>> + * >>>> + * Context: The caller holds the page table lock. The PTEs map consecutive >>>> + * pages that belong to the same folio. The PTEs are all in the same PMD. >>>> + */ >>>> +static inline int clear_flush_young_ptes(struct vm_area_struct *vma, >>>> + unsigned long addr, pte_t *ptep, unsigned int nr) >>>> +{ >>>> + int young = 0; >>>> + >>>> + for (;;) { >>>> + young |= ptep_clear_flush_young(vma, addr, ptep); >>>> + if (--nr == 0) >>>> + break; >>>> + ptep++; >>>> + addr += PAGE_SIZE; >>>> + } >>>> + >>>> + return young; >>>> +} >>>> +#endif >>> >>> We might have an opportunity to batch the TLB synchronization, >>> using flush_tlb_range() instead of calling flush_tlb_page() >>> one by one. Not sure the benefit would be significant though, >>> especially if only one entry among nr has the young bit set. >> >> Yes. In addition, this will involve many architectures’ implementations >> and their differing TLB flush mechanisms, so it’s difficult to make a >> reasonable per-architecture measurement. If any architecture has a more >> efficient flush method, I’d prefer to implement an architecture‑specific >> clear_flush_young_ptes(). > > Right! Since TLBI is usually quite expensive, I wonder if a generic > implementation for architectures lacking clear_flush_young_ptes() > might benefit from something like the below (just a very rough idea): > > int clear_flush_young_ptes(struct vm_area_struct *vma, > unsigned long addr, pte_t *ptep, unsigned int nr) > { > unsigned long curr_addr = addr; > int young = 0; > > while (nr--) { > young |= ptep_test_and_clear_young(vma, curr_addr, ptep); > ptep++; > curr_addr += PAGE_SIZE; > } > > if (young) > flush_tlb_range(vma, addr, curr_addr); > return young; > } I understand your point. I’m concerned that I can’t test this patch on every architecture to validate the benefits. Anyway, let me try this on my X86 machine first.