linux-kernel - Re: [PATCH 06/25] mm/arm64: Use mm_fault

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Date:   Tue, 16 Jun 2020 08:43:08 +0100
From:   Will Deacon <will@...nel.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 06/25] mm/arm64: Use mm_fault_accounting()

On Mon, Jun 15, 2020 at 06:15:48PM -0400, Peter Xu wrote:
> Use the new mm_fault_accounting() helper for page fault accounting.
> 
> CC: Catalin Marinas <catalin.marinas@....com>
> CC: Will Deacon <will@...nel.org>
> CC: linux-arm-kernel@...ts.infradead.org
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  arch/arm64/mm/fault.c | 17 ++---------------
>  1 file changed, 2 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index c9cedc0432d2..09af7d7a60ec 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -484,8 +484,6 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
>  					 addr, esr, regs);
>  	}
>  
> -	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr);
> -
>  	/*
>  	 * As per x86, we may deadlock here. However, since the kernel only
>  	 * validly references user space from well defined areas of the code,
> @@ -535,20 +533,9 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
>  			      VM_FAULT_BADACCESS)))) {
>  		/*
>  		 * Major/minor page fault accounting is only done
> -		 * once. If we go through a retry, it is extremely
> -		 * likely that the page will be found in page cache at
> -		 * that point.
> +		 * once.
>  		 */
> -		if (major) {
> -			current->maj_flt++;
> -			perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1, regs,
> -				      addr);
> -		} else {
> -			current->min_flt++;
> -			perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1, regs,
> -				      addr);
> -		}
> -
> +		mm_fault_accounting(current, regs, address, major);

Please can you explain why it's ok to move the PERF_COUNT_SW_PAGE_FAULTS
update like this? Seems like a user-visible change to me, so some
justification would really help.

Will