linux-kernel - Re: [PATCH 17/26] mm/riscv: Use general page fault accounting

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Date:   Sat, 11 Jul 2020 12:43:47 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     peterx@...hat.com
CC:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        peterx@...hat.com, akpm@...ux-foundation.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        gerald.schaefer@...ibm.com, aarcange@...hat.com, will@...nel.org,
        mpe@...erman.id.au, Paul Walmsley <paul.walmsley@...ive.com>,
        aou@...s.berkeley.edu, linux-riscv@...ts.infradead.org
Subject:     Re: [PATCH 17/26] mm/riscv: Use general page fault accounting

On Fri, 26 Jun 2020 15:36:25 PDT (-0700), peterx@...hat.com wrote:
> Use the general page fault accounting by passing regs into handle_mm_fault().
> It naturally solve the issue of multiple page fault accounting when page fault
> retry happened.
>
> CC: Paul Walmsley <paul.walmsley@...ive.com>
> CC: Palmer Dabbelt <palmer@...belt.com>
> CC: Albert Ou <aou@...s.berkeley.edu>
> CC: linux-riscv@...ts.infradead.org
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  arch/riscv/mm/fault.c | 16 +---------------
>  1 file changed, 1 insertion(+), 15 deletions(-)
>
> diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
> index 677ee1bb11ac..e796ba02b572 100644
> --- a/arch/riscv/mm/fault.c
> +++ b/arch/riscv/mm/fault.c
> @@ -110,7 +110,7 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
>  	 * make sure we exit gracefully rather than endlessly redo
>  	 * the fault.
>  	 */
> -	fault = handle_mm_fault(vma, addr, flags, NULL);
> +	fault = handle_mm_fault(vma, addr, flags, regs);
>
>  	/*
>  	 * If we need to retry but a fatal signal is pending, handle the
> @@ -128,21 +128,7 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
>  		BUG();
>  	}
>
> -	/*
> -	 * Major/minor page fault accounting is only done on the
> -	 * initial attempt. If we go through a retry, it is extremely
> -	 * likely that the page will be found in page cache at that point.
> -	 */
>  	if (flags & FAULT_FLAG_ALLOW_RETRY) {
> -		if (fault & VM_FAULT_MAJOR) {
> -			tsk->maj_flt++;
> -			perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ,
> -				      1, regs, addr);
> -		} else {
> -			tsk->min_flt++;
> -			perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN,
> -				      1, regs, addr);
> -		}
>  		if (fault & VM_FAULT_RETRY) {
>  			flags |= FAULT_FLAG_TRIED;

This still slightly changes the accounting numbers, but I don't think it does
so in a way that's meaningful enough to care about.  SIGBUS is the only one
that might happen frequently enough to notice, I doubt anyone cares about
whether faults are accounted for during OOM.

Acked-by: Palmer Dabbelt <palmerdabbelt@...gle.com>

Thanks!