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Message-ID: <c5df1f9f-952a-4e83-9ae0-bd0a83dedd36@arm.com>
Date: Wed, 24 Dec 2025 14:07:34 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
david@...nel.org, catalin.marinas@....com, will@...nel.org
Cc: lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, vbabka@...e.cz,
rppt@...nel.org, surenb@...gle.com, mhocko@...e.com, riel@...riel.com,
harry.yoo@...cle.com, jannh@...gle.com, willy@...radead.org,
baohua@...nel.org, dev.jain@....com, linux-mm@...ck.org,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/5] arm64: mm: support batch clearing of the young
flag for large folios
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 <baolin.wang@...ux.alibaba.com>
> ---
> 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.
> }
>
> #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?
Thanks,
Ryan
> }
>
> return young;
> }
> -EXPORT_SYMBOL_GPL(contpte_ptep_clear_flush_young);
> +EXPORT_SYMBOL_GPL(contpte_clear_flush_young_ptes);
>
> void contpte_wrprotect_ptes(struct mm_struct *mm, unsigned long addr,
> pte_t *ptep, unsigned int nr)
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