linux-kernel - Re: [PATCH v5 18/25] mm/s390: Use general page fault accounting

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Date:   Wed, 8 Jul 2020 07:49:47 +0200
From:   Alexander Gordeev <agordeev@...ux.ibm.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Will Deacon <will@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        linux-s390@...r.kernel.org
Subject: Re: [PATCH v5 18/25] mm/s390: Use general page fault accounting

On Tue, Jul 07, 2020 at 06:50:14PM -0400, Peter Xu 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: Heiko Carstens <heiko.carstens@...ibm.com>
> CC: Vasily Gorbik <gor@...ux.ibm.com>
> CC: Christian Borntraeger <borntraeger@...ibm.com>
> CC: linux-s390@...r.kernel.org
> Reviewed-by: Gerald Schaefer <gerald.schaefer@...ibm.com>
> Acked-by: Gerald Schaefer <gerald.schaefer@...ibm.com>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  arch/s390/mm/fault.c | 16 +---------------
>  1 file changed, 1 insertion(+), 15 deletions(-)
> 
> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> index fc14df0b4d6e..9aa201df2e94 100644
> --- a/arch/s390/mm/fault.c
> +++ b/arch/s390/mm/fault.c
> @@ -478,7 +478,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
>  	 * make sure we exit gracefully rather than endlessly redo
>  	 * the fault.
>  	 */
> -	fault = handle_mm_fault(vma, address, flags, NULL);
> +	fault = handle_mm_fault(vma, address, flags, regs);
>  	if (fault_signal_pending(fault, regs)) {
>  		fault = VM_FAULT_SIGNAL;
>  		if (flags & FAULT_FLAG_RETRY_NOWAIT)
> @@ -488,21 +488,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
>  	if (unlikely(fault & VM_FAULT_ERROR))
>  		goto out_up;
>  
> -	/*
> -	 * 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) {

Shouldn't this check ^^^ be dropped as well?

Since commit 4064b9827063 ("mm: allow VM_FAULT_RETRY for multiple times")
FAULT_FLAG_ALLOW_RETRY never gets unset, so no need to check..

> -		if (fault & VM_FAULT_MAJOR) {
> -			tsk->maj_flt++;
> -			perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,
> -				      regs, address);
> -		} else {
> -			tsk->min_flt++;
> -			perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1,
> -				      regs, address);
> -		}
>  		if (fault & VM_FAULT_RETRY) {
>  			if (IS_ENABLED(CONFIG_PGSTE) && gmap &&
>  			    (flags & FAULT_FLAG_RETRY_NOWAIT)) {
> -- 
> 2.26.2
>