[PATCH 2/7] arm64: mm: accelerate pagefault when VM_FAULT_BADACCESS

[Suren Baghdasaryan]

On Tue, Apr 2, 2024 at 12:53 AM Kefeng Wang <wangkefeng.wang at huawei.com> wrote:
>
> The vm_flags of vma already checked under per-VMA lock, if it is a
> bad access, directly set fault to VM_FAULT_BADACCESS and handle error,
> no need to lock_mm_and_find_vma() and check vm_flags again, the latency
> time reduce 34% in lmbench 'lat_sig -P 1 prot lat_sig'.

The change makes sense to me. Per-VMA lock is enough to keep
vma->vm_flags stable, so no need to retry with mmap_lock.

>
> Signed-off-by: Kefeng Wang <wangkefeng.wang at huawei.com>

Reviewed-by: Suren Baghdasaryan <surenb at google.com>

> ---
>  arch/arm64/mm/fault.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 9bb9f395351a..405f9aa831bd 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -572,7 +572,9 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr,
>
>         if (!(vma->vm_flags & vm_flags)) {
>                 vma_end_read(vma);
> -               goto lock_mmap;
> +               fault = VM_FAULT_BADACCESS;
> +               count_vm_vma_lock_event(VMA_LOCK_SUCCESS);

nit: VMA_LOCK_SUCCESS accounting here seems correct to me but
unrelated to the main change. Either splitting into a separate patch
or mentioning this additional fixup in the changelog would be helpful.

> +               goto done;
>         }
>         fault = handle_mm_fault(vma, addr, mm_flags | FAULT_FLAG_VMA_LOCK, regs);
>         if (!(fault & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)))
> --
> 2.27.0
>