From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 2978B2512C8 for ; Fri, 2 Jan 2026 12:12:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767355934; cv=none; b=b/5q820sqm9SjGOUdLBt3IOPiuNGrBCCFvt9MNTrMbggAtagfGLovQIP1sNQeXqLXMXsVWjlo0db7viCMh7OZ41e+Ug1c9ApfkaxMoi77EIHW9rKTbmJRob1tsTlKZ1Tc0klNNdOBtVupxtToO2Hl3DkbBVLuKR0GE1aYjiGrpM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767355934; c=relaxed/simple; bh=iZmKMs1iToXTjH6TQOIlW4bGW7BqZYUvpAv23skIdyM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=qkVdNlTvWauCL7K+54kz1QLIQ9m2m9hcL8fQZNmM01sZRboGGxCsXUfZXfzqy5kdAsRv87bDertBAuEtPc0GRakz+DabY6uZ4UyWFb53HJwUReyA4ADKT4miI3W36LxMaQyQVh7dRAWWkyGUoyAtAezSnQ3/xyB8H84Kzqjta/Q= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 5EC6E497; Fri, 2 Jan 2026 04:12:03 -0800 (PST) Received: from [10.57.94.221] (unknown [10.57.94.221]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 3072E3F63F; Fri, 2 Jan 2026 04:12:07 -0800 (PST) Message-ID: <85094ce2-21a8-4d25-bd96-7d809c5daa3b@arm.com> Date: Fri, 2 Jan 2026 12:12:05 +0000 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 v4 3/5] arm64: mm: support batch clearing of the young flag for large folios Content-Language: en-GB To: Baolin Wang , akpm@linux-foundation.org, david@kernel.org, catalin.marinas@arm.com, will@kernel.org Cc: lorenzo.stoakes@oracle.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: <3b427d9010a6d52f2b91342760f12be097d21cf6.1766455378.git.baolin.wang@linux.alibaba.com> <02239ca7-9701-4bfa-af0f-dcf0d05a3e89@linux.alibaba.com> From: Ryan Roberts In-Reply-To: <02239ca7-9701-4bfa-af0f-dcf0d05a3e89@linux.alibaba.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 25/12/2025 02:48, Baolin Wang wrote: > > > On 2025/12/24 22:07, Ryan Roberts wrote: >> On 23/12/2025 05:48, Baolin Wang wrote: >>> Currently, contpte_ptep_test_and_clear_young() and >>> contpte_ptep_clear_flush_young() >>> only clear the young flag and flush TLBs for PTEs within the contiguous range. >>> To support batch PTE operations for other sized large folios in the following >>> patches, adding a new parameter to specify the number of PTEs that map >>> consecutive >>> pages of the same large folio in a single VMA and a single page table. >>> >>> While we are at it, rename the functions to maintain consistency with other >>> contpte_*() functions. >>> >>> Signed-off-by: Baolin Wang >>> --- >>>   arch/arm64/include/asm/pgtable.h | 12 ++++++------ >>>   arch/arm64/mm/contpte.c          | 33 ++++++++++++++++++-------------- >>>   2 files changed, 25 insertions(+), 20 deletions(-) >>> >>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h >>> index 445e18e92221..d5fbe72e820a 100644 >>> --- a/arch/arm64/include/asm/pgtable.h >>> +++ b/arch/arm64/include/asm/pgtable.h >>> @@ -1648,10 +1648,10 @@ extern void contpte_clear_full_ptes(struct mm_struct >>> *mm, unsigned long addr, >>>   extern pte_t contpte_get_and_clear_full_ptes(struct mm_struct *mm, >>>                   unsigned long addr, pte_t *ptep, >>>                   unsigned int nr, int full); >>> -extern int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma, >>> -                unsigned long addr, pte_t *ptep); >>> -extern int contpte_ptep_clear_flush_young(struct vm_area_struct *vma, >>> -                unsigned long addr, pte_t *ptep); >>> +int contpte_test_and_clear_young_ptes(struct vm_area_struct *vma, >>> +                unsigned long addr, pte_t *ptep, unsigned int nr); >>> +int contpte_clear_flush_young_ptes(struct vm_area_struct *vma, >>> +                unsigned long addr, pte_t *ptep, unsigned int nr); >>>   extern void contpte_wrprotect_ptes(struct mm_struct *mm, unsigned long addr, >>>                   pte_t *ptep, unsigned int nr); >>>   extern int contpte_ptep_set_access_flags(struct vm_area_struct *vma, >>> @@ -1823,7 +1823,7 @@ static inline int ptep_test_and_clear_young(struct >>> vm_area_struct *vma, >>>       if (likely(!pte_valid_cont(orig_pte))) >>>           return __ptep_test_and_clear_young(vma, addr, ptep); >>>   -    return contpte_ptep_test_and_clear_young(vma, addr, ptep); >>> +    return contpte_test_and_clear_young_ptes(vma, addr, ptep, CONT_PTES); >> >> As per your fixup patch, I agree that nr should be 1 here, not CONT_PTES. > > Yes. > >>>   } >>>     #define __HAVE_ARCH_PTEP_CLEAR_YOUNG_FLUSH >>> @@ -1835,7 +1835,7 @@ static inline int ptep_clear_flush_young(struct >>> vm_area_struct *vma, >>>       if (likely(!pte_valid_cont(orig_pte))) >>>           return __ptep_clear_flush_young(vma, addr, ptep); >>>   -    return contpte_ptep_clear_flush_young(vma, addr, ptep); >>> +    return contpte_clear_flush_young_ptes(vma, addr, ptep, CONT_PTES); >> >> And same here. >> >>>   } >>>     #define wrprotect_ptes wrprotect_ptes >>> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c >>> index e4ddeb46f25d..b929a455103f 100644 >>> --- a/arch/arm64/mm/contpte.c >>> +++ b/arch/arm64/mm/contpte.c >>> @@ -508,8 +508,9 @@ pte_t contpte_get_and_clear_full_ptes(struct mm_struct *mm, >>>   } >>>   EXPORT_SYMBOL_GPL(contpte_get_and_clear_full_ptes); >>>   -int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma, >>> -                    unsigned long addr, pte_t *ptep) >>> +int contpte_test_and_clear_young_ptes(struct vm_area_struct *vma, >>> +                    unsigned long addr, pte_t *ptep, >>> +                    unsigned int nr) >>>   { >>>       /* >>>        * ptep_clear_flush_young() technically requires us to clear the access >>> @@ -518,41 +519,45 @@ int contpte_ptep_test_and_clear_young(struct >>> vm_area_struct *vma, >>>        * contig range when the range is covered by a single folio, we can get >>>        * away with clearing young for the whole contig range here, so we avoid >>>        * having to unfold. >>> +     * >>> +     * The 'nr' means consecutive (present) PTEs that map consecutive pages >>> +     * of the same large folio in a single VMA and a single page table. >>>        */ >>>   +    unsigned long end = addr + nr * PAGE_SIZE; >>>       int young = 0; >>> -    int i; >>>   -    ptep = contpte_align_down(ptep); >>> -    addr = ALIGN_DOWN(addr, CONT_PTE_SIZE); >>> - >>> -    for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE) >>> +    ptep = contpte_align_addr_ptep(&addr, &end, ptep, nr); >>> +    for (; addr != end; ptep++, addr += PAGE_SIZE) >>>           young |= __ptep_test_and_clear_young(vma, addr, ptep); >>>         return young; >>>   } >>> -EXPORT_SYMBOL_GPL(contpte_ptep_test_and_clear_young); >>> +EXPORT_SYMBOL_GPL(contpte_test_and_clear_young_ptes); >>>   -int contpte_ptep_clear_flush_young(struct vm_area_struct *vma, >>> -                    unsigned long addr, pte_t *ptep) >>> +int contpte_clear_flush_young_ptes(struct vm_area_struct *vma, >>> +                unsigned long addr, pte_t *ptep, >>> +                unsigned int nr) >>>   { >>>       int young; >>>   -    young = contpte_ptep_test_and_clear_young(vma, addr, ptep); >>> +    young = contpte_test_and_clear_young_ptes(vma, addr, ptep, nr); >>>         if (young) { >>> +        unsigned long end = addr + nr * PAGE_SIZE; >>> + >>> +        contpte_align_addr_ptep(&addr, &end, ptep, nr); >>>           /* >>>            * See comment in __ptep_clear_flush_young(); same rationale for >>>            * eliding the trailing DSB applies here. >>>            */ >>> -        addr = ALIGN_DOWN(addr, CONT_PTE_SIZE); >>> -        __flush_tlb_range_nosync(vma->vm_mm, addr, addr + CONT_PTE_SIZE, >>> +        __flush_tlb_range_nosync(vma->vm_mm, addr, end, >>>                        PAGE_SIZE, true, 3); >> >> Hmm... The requirement is that we must flush the _page_ if clearing access for a >> pte that does not have the contiguous bit set, or we must flush the _contpte >> block_ if clearing access for a pte that does have the contiguous bit set. >> >> With your changes, you may call for a large range that covers multiple contpte >> blocks but only has a single pte in a single contpte block for which the access >> bit was previously set. But that will cause flushing the TLB for the full range. >> Could this cause a performance issue? Yes, no, maybe... I think it's unlikely >> but I wouldn't rule it out in some edge case. >> >> I wonder if it's better to track the sub-ranges where access was cleared and >> only issue tlbi for those sub-ranges? Probably just keep it simple (the way you >> have done it) until/unless we see an actual problem? > > Good question. Indeed, as you said, we flush the TLB per folio now, which might > increase the flush range. However, I think this approach is relatively > reasonable for now. > > First, the mm-core also tracks the access status per folio, and it's really > unnecessary to add excessive complexity to track the access status of sub-pages > (or sub-ranges). I can already imagine that tracking the access status for each > cont-block range as well as for non-cont pages across the entire large folio > range, which can be too complicated. > > Second, __flush_tlb_range_nosync() is a lightweight flush. I quickly ran a > measurement on my machine and found that the overhead of > __flush_tlb_range_nosync() barely changes between nr=16 and nr=256 (both are > around 40 ns). I'm not concerned about the direct cost of the flush; I agree it should be lightweight given we elide the trailing DSB. (although there is a possible case on older HW that doesn't support TLBI-by-range where this will be converted to multiple TLBI-by-page instructions and that can cause stalls if there are too many of them). My concern was the opportunity cost of evicting the entries for all the non-accessed parts of the folio from the TLB. But of course, I'm talking nonsense because the architecture does not allow caching non-accessed entries in the TLB. So doesn't sound like a problem; I think we can ignore this. Sorry for the noise. > > Therefore, I would still prefer to keep the logic here simple. Agreed. Thanks, Ryan