linux-kernel - Re: [PATCH v4 2/4] riscv: Improve flush_tlb

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Message-ID: <CAHVXubiaYwQ3PeiA-688cKuBscdEb5QpNerbYSMWYqiXCm-FRw@mail.gmail.com>
Date:   Wed, 18 Oct 2023 13:32:24 +0200
From:   Alexandre Ghiti <alexghiti@...osinc.com>
To:     Samuel Holland <samuel.holland@...ive.com>
Cc:     Andrew Jones <ajones@...tanamicro.com>,
        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 v4 2/4] riscv: Improve flush_tlb_range() for hugetlb pages

On Mon, Oct 9, 2023 at 7:53 PM Samuel Holland <samuel.holland@...ive.com> wrote:
>
> Hi Alex,
>
> On 2023-09-11 8:12 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>
> > Reviewed-by: Andrew Jones <ajones@...tanamicro.com>
> > ---
> >  arch/riscv/mm/tlbflush.c | 39 ++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 38 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> > index fa03289853d8..5bda6d4fed90 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 +148,43 @@ 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;
> > +
> > +     stride_size = is_vm_hugetlb_page(vma) ?
> > +                             huge_page_size(hstate_vma(vma)) :
> > +                             PAGE_SIZE;
> > +
> > +#ifdef CONFIG_RISCV_ISA_SVNAPOT
> > +     /*
> > +      * 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()) {
>
> This whole block should probably go inside the is_vm_hugetlb_page(vma) check,
> since we have to perform that check anyway.

Yes, you're right.

>
> > +             unsigned long order, napot_size;
> > +
> > +             for_each_napot_order(order) {
> > +                     napot_size = napot_cont_size(order);
> > +
> > +                     if (stride_size != napot_size)
> > +                             continue;
> > +
> > +                     if (napot_size >= PGDIR_SIZE)
>
> Can you check stride_size here directly, and drop the loop? We should be able to
> assume that the huge page size is valid. Non-NAPOT hugepages will hit one of the
> equal-to cases below, which is fine.

Yes, again, you're right.

I'll respin a new version now, let it go through our CI and send it tomorrow,

Thanks,

Alex

>
> Regards,
> Samuel
>
> > +                             stride_size = PGDIR_SIZE;
> > +                     else if (napot_size >= P4D_SIZE)
> > +                             stride_size = P4D_SIZE;
> > +                     else if (napot_size >= PUD_SIZE)
> > +                             stride_size = PUD_SIZE;
> > +                     else if (napot_size >= PMD_SIZE)
> > +                             stride_size = PMD_SIZE;
> > +                     else
> > +                             stride_size = PAGE_SIZE;
> > +
> > +                     break;
> > +             }
> > +     }
> > +#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,
>