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

From: Catalin Marinas <[email-masked]>
Date: Wed, 3 Apr 2024 19:32:25 +0100

On Tue, Apr 02, 2024 at 03:51:37PM +0800, Kefeng Wang 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'.
> 
> Signed-off-by: Kefeng Wang <[email-masked]>
> ---
>  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);
> +		goto done;
>  	}
>  	fault = handle_mm_fault(vma, addr, mm_flags | FAULT_FLAG_VMA_LOCK, regs);
>  	if (!(fault & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)))

I think this makes sense. A concurrent modification of vma->vm_flags
(e.g. mprotect()) would do a vma_start_write(), so no need to recheck
again with the mmap lock held.

Reviewed-by: Catalin Marinas <[email-masked]>