Re: [PATCH 4/7] powerpc: mm: accelerate pagefault when badaccess

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Apr 2, 2024 at 12:53 AM Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
>
> The vm_flags of vma already checked under per-VMA lock, if it is a
> bad access, directly handle error and return, there is no need to
> lock_mm_and_find_vma() and check vm_flags again.
>
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>

Code-wise looks correct to me and almost identical to x86 change but
someone with more experience with this architecture should review.

> ---
>  arch/powerpc/mm/fault.c | 33 ++++++++++++++++++++-------------
>  1 file changed, 20 insertions(+), 13 deletions(-)
>
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index 53335ae21a40..215690452495 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -71,23 +71,26 @@ static noinline int bad_area_nosemaphore(struct pt_regs *regs, unsigned long add
>         return __bad_area_nosemaphore(regs, address, SEGV_MAPERR);
>  }
>
> -static int __bad_area(struct pt_regs *regs, unsigned long address, int si_code)
> +static int __bad_area(struct pt_regs *regs, unsigned long address, int si_code,
> +                     struct mm_struct *mm, struct vm_area_struct *vma)
>  {
> -       struct mm_struct *mm = current->mm;
>
>         /*
>          * Something tried to access memory that isn't in our memory map..
>          * Fix it, but check if it's kernel or user first..
>          */
> -       mmap_read_unlock(mm);
> +       if (mm)
> +               mmap_read_unlock(mm);
> +       else
> +               vma_end_read(vma);
>
>         return __bad_area_nosemaphore(regs, address, si_code);
>  }
>
>  static noinline int bad_access_pkey(struct pt_regs *regs, unsigned long address,
> +                                   struct mm_struct *mm,
>                                     struct vm_area_struct *vma)
>  {
> -       struct mm_struct *mm = current->mm;
>         int pkey;
>
>         /*
> @@ -109,7 +112,10 @@ static noinline int bad_access_pkey(struct pt_regs *regs, unsigned long address,
>          */
>         pkey = vma_pkey(vma);
>
> -       mmap_read_unlock(mm);
> +       if (mm)
> +               mmap_read_unlock(mm);
> +       else
> +               vma_end_read(vma);
>
>         /*
>          * If we are in kernel mode, bail out with a SEGV, this will
> @@ -124,9 +130,10 @@ static noinline int bad_access_pkey(struct pt_regs *regs, unsigned long address,
>         return 0;
>  }
>
> -static noinline int bad_access(struct pt_regs *regs, unsigned long address)
> +static noinline int bad_access(struct pt_regs *regs, unsigned long address,
> +                              struct mm_struct *mm, struct vm_area_struct *vma)
>  {
> -       return __bad_area(regs, address, SEGV_ACCERR);
> +       return __bad_area(regs, address, SEGV_ACCERR, mm, vma);
>  }
>
>  static int do_sigbus(struct pt_regs *regs, unsigned long address,
> @@ -479,13 +486,13 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
>
>         if (unlikely(access_pkey_error(is_write, is_exec,
>                                        (error_code & DSISR_KEYFAULT), vma))) {
> -               vma_end_read(vma);
> -               goto lock_mmap;
> +               count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
> +               return bad_access_pkey(regs, address, NULL, vma);
>         }
>
>         if (unlikely(access_error(is_write, is_exec, vma))) {
> -               vma_end_read(vma);
> -               goto lock_mmap;
> +               count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
> +               return bad_access(regs, address, NULL, vma);
>         }
>
>         fault = handle_mm_fault(vma, address, flags | FAULT_FLAG_VMA_LOCK, regs);
> @@ -521,10 +528,10 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
>
>         if (unlikely(access_pkey_error(is_write, is_exec,
>                                        (error_code & DSISR_KEYFAULT), vma)))
> -               return bad_access_pkey(regs, address, vma);
> +               return bad_access_pkey(regs, address, mm, vma);
>
>         if (unlikely(access_error(is_write, is_exec, vma)))
> -               return bad_access(regs, address);
> +               return bad_access(regs, address, mm, vma);
>
>         /*
>          * If for any reason at all we couldn't handle the fault,
> --
> 2.27.0
>