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Mon, 09 Feb 2026 17:25:34 +0800 Message-ID: Date: Mon, 9 Feb 2026 17:25:33 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 1/5] mm: rmap: support batched checks of the references for large folios To: "David Hildenbrand (Arm)" , akpm@linux-foundation.org, catalin.marinas@arm.com, will@kernel.org Cc: 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, baohua@kernel.org, dev.jain@arm.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <18b3eb9c730d16756e5d23c7be22efe2f6219911.1766631066.git.baolin.wang@linux.alibaba.com> <3d5cb9a4-6604-4302-a110-3d8ff91baa56@kernel.org> <44453a4c-50a2-4e7e-9d2a-ebf973ccf6b7@linux.alibaba.com> From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 00D4B10000A X-Stat-Signature: 4pqoosxcrcmyjxzn8ufmfkhteorca633 X-Rspam-User: X-Rspamd-Server: rspam02 X-HE-Tag: 1770629140-525645 X-HE-Meta: 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 EmHh7dfi d7mmkyeSUTjyNiyq9+MN+lVlZFqQzRboAx5c3rlRR7V46mFn2L/UJr8YqQzHH4WXkM3hGSsB+PpYJvwKnshiBEuIJyCFePqG+xYkdj1iKCysXC9RncJkBlX8Nycd/IyqjHriELYa6AFBGSzQVuNuZvUE/ixA0u1mRZAzcj3N/CxC5M+nz25Bwoo0eCzYbvBwLYA2ok/YsnpRciO5/KeqPZtpQQ+rqjRwwj7ig/Fu5eq6lTdklMi2gynE6r7ea5aYvWM5WUgGvqVevGbm0PPNmpMzPdOhZNZRdRCuLQfuwK9SL7G6gOVq30OaTDJ5PHSrnCWpER27htZrF8XiMxCNhCjncXaGp1yiEAZIBECj2LN1O8VKDSp/stNMtwnLorYSFM5wFjzlXyBavhN8yFdnaRPjX3/tvTbS8nGNy 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 2/9/26 5:20 PM, David Hildenbrand (Arm) wrote: > On 2/9/26 10:14, Baolin Wang wrote: >> >> >> On 2/9/26 4:49 PM, David Hildenbrand (Arm) wrote: >>> On 12/26/25 07:07, 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: Ryan Roberts >>>> Signed-off-by: Baolin Wang >>>> --- >>>>   include/linux/mmu_notifier.h |  9 +++++---- >>>>   include/linux/pgtable.h      | 31 +++++++++++++++++++++++++++++++ >>>>   mm/rmap.c                    | 31 ++++++++++++++++++++++++++++--- >>>>   3 files changed, 64 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;                            \ >>>>   }) >>> >>> Man that's ugly, Not your fault, but can this possibly be turned into >>> an inline function in a follow-up patch. >> >> Yes, the cleanup of these macros is already in my follow-up patch set. >> >>>> +#ifndef clear_flush_young_ptes >>>> +/** >>>> + * clear_flush_young_ptes - Clear the access bit and perform a TLB >>>> flush for PTEs >>>> + *                that map consecutive pages of the same folio. >>> >>> With clear_young_dirty_ptes() description in mind, this should >>> probably be "Mark PTEs that map consecutive pages of the same folio >>> as clean and flush the TLB" ? >> >> IMO, “clean” is confusing here, as it sounds like clear the dirty bit >> to make the folio clean. > > "as old", sorry, I used the wrong part of the description. OK. >>>> + * @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) >>> >>> Two-tab alignment on second+ line like all similar functions here. >> >> Sure. >> >>>> +{ >>>> +    int i, young = 0; >>>> + >>>> +    for (i = 0; i < nr; ++i, ++ptep, addr += PAGE_SIZE) >>>> +        young |= ptep_clear_flush_young(vma, addr, ptep); >>>> + >>> >>> Why don't we use a similar loop we use in clear_young_dirty_ptes() or >>> clear_full_ptes() etc? It's not only consistent but also optimizes >>> out the first check for nr. >>> for (;;) { >>>      young |= ptep_clear_flush_young(vma, addr, ptep); >>>      if (--nr == 0) >>>          break; >>>      ptep++; >>>      addr += PAGE_SIZE; >>> } >> >> We’ve discussed this loop pattern before [1], and it seems that people >> prefer the ‘for (;;)’ loop. Do you have a strong preference for >> changing it back? > > Yes, to make all such helpers look consistent. Note that your version > was also not consistent with the other variants. > > Ryans point was about avoiding two ptep_clear_flush_young() calls, which > the for(;;) avoids as well. Actually my v2[1] is following the previous pattern, anyway let me change it back. [1] https://lore.kernel.org/all/545dba5e899634bc6c8ca782417d16fef3bd049f.1765439381.git.baolin.wang@linux.alibaba.com/