linux-kernel - Re: [PATCH v5 2/4] riscv: Improve flush_tlb

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Date:   Sat, 28 Oct 2023 13:52:41 -0500
From:   Samuel Holland <samuel.holland@...ive.com>
To:     Alexandre Ghiti <alexghiti@...osinc.com>
Cc:     Will Deacon <will@...nel.org>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nick Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mayuresh Chitale <mchitale@...tanamicro.com>,
        Vincent Chen <vincent.chen@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>, linux-arch@...r.kernel.org,
        linux-mm@...ck.org, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Samuel Holland <samuel@...lland.org>,
        Lad Prabhakar <prabhakar.csengg@...il.com>
Subject: Re: [PATCH v5 2/4] riscv: Improve flush_tlb_range() for hugetlb pages

On 2023-10-19 9:01 AM, Alexandre Ghiti wrote:
> flush_tlb_range() uses a fixed stride of PAGE_SIZE and in its current form,
> when a hugetlb mapping needs to be flushed, flush_tlb_range() flushes the
> whole tlb: so set a stride of the size of the hugetlb mapping in order to
> only flush the hugetlb mapping. However, if the hugepage is a NAPOT region,
> all PTEs that constitute this mapping must be invalidated, so the stride
> size must actually be the size of the PTE.
> 
> Note that THPs are directly handled by flush_pmd_tlb_range().
> 
> Signed-off-by: Alexandre Ghiti <alexghiti@...osinc.com>
> ---
>  arch/riscv/mm/tlbflush.c | 31 ++++++++++++++++++++++++++++++-
>  1 file changed, 30 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> index fa03289853d8..5933744df91a 100644
> --- a/arch/riscv/mm/tlbflush.c
> +++ b/arch/riscv/mm/tlbflush.c
> @@ -3,6 +3,7 @@
>  #include <linux/mm.h>
>  #include <linux/smp.h>
>  #include <linux/sched.h>
> +#include <linux/hugetlb.h>
>  #include <asm/sbi.h>
>  #include <asm/mmu_context.h>
>  
> @@ -147,7 +148,35 @@ void flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
>  void flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
>  		     unsigned long end)
>  {
> -	__flush_tlb_range(vma->vm_mm, start, end - start, PAGE_SIZE);
> +	unsigned long stride_size;
> +
> +	if (!is_vm_hugetlb_page(vma)) {
> +		stride_size = PAGE_SIZE;
> +	} else {
> +		stride_size = huge_page_size(hstate_vma(vma));
> +
> +#ifdef CONFIG_RISCV_ISA_SVNAPOT

There is a fallback implementation of has_svnapot(), so you do not need this
preprocessor check. With that removed:

Reviewed-by: Samuel Holland <samuel.holland@...ive.com>

> +		/*
> +		 * As stated in the privileged specification, every PTE in a
> +		 * NAPOT region must be invalidated, so reset the stride in that
> +		 * case.
> +		 */
> +		if (has_svnapot()) {
> +			if (stride_size >= PGDIR_SIZE)
> +				stride_size = PGDIR_SIZE;
> +			else if (stride_size >= P4D_SIZE)
> +				stride_size = P4D_SIZE;

As a side note, and this is probably premature optimization... PGDIR_SIZE and
P4D_SIZE check pgtable_l{4,5}_enabled. That's not really necessary here, since
we are just trying to round down, and there won't be any higher-order hugepages
if those paging levels are disabled.

> +			else if (stride_size >= PUD_SIZE)
> +				stride_size = PUD_SIZE;
> +			else if (stride_size >= PMD_SIZE)
> +				stride_size = PMD_SIZE;
> +			else
> +				stride_size = PAGE_SIZE;
> +		}
> +#endif
> +	}
> +
> +	__flush_tlb_range(vma->vm_mm, start, end - start, stride_size);
>  }
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  void flush_pmd_tlb_range(struct vm_area_struct *vma, unsigned long start,