linux-kernel - Re: [PATCH 19/25] mm/s390: Use mm_fault

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Date:   Tue, 16 Jun 2020 17:59:33 +0200
From:   Alexander Gordeev <agordeev@...ux.ibm.com>
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>,
        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 19/25] mm/s390: Use mm_fault_accounting()

On Mon, Jun 15, 2020 at 06:23:02PM -0400, Peter Xu wrote:
> Use the new mm_fault_accounting() helper for page fault accounting.
> 
> Avoid doing page fault accounting multiple times if the page fault is retried.
> 
> 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
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  arch/s390/mm/fault.c | 21 +++++----------------
>  1 file changed, 5 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> index dedc28be27ab..8ca207635b59 100644
> --- a/arch/s390/mm/fault.c
> +++ b/arch/s390/mm/fault.c
> @@ -392,7 +392,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
>  	unsigned long trans_exc_code;
>  	unsigned long address;
>  	unsigned int flags;
> -	vm_fault_t fault;
> +	vm_fault_t fault, major = 0;
> 
>  	tsk = current;
>  	/*
> @@ -428,7 +428,6 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
>  	}
> 
>  	address = trans_exc_code & __FAIL_ADDR_MASK;
> -	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
>  	flags = FAULT_FLAG_DEFAULT;
>  	if (user_mode(regs))
>  		flags |= FAULT_FLAG_USER;
> @@ -480,6 +479,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
>  	 * the fault.
>  	 */
>  	fault = handle_mm_fault(vma, address, flags);
> +	major |= fault & VM_FAULT_MAJOR;
>  	if (fault_signal_pending(fault, regs)) {
>  		fault = VM_FAULT_SIGNAL;
>  		if (flags & FAULT_FLAG_RETRY_NOWAIT)
> @@ -489,21 +489,7 @@ 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) {
> -		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)) {

Seems like the call to mm_fault_accounting() will be missed if
we entered here with FAULT_FLAG_RETRY_NOWAIT flag set, since it
jumps to "out_up"...

> @@ -519,6 +505,9 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
>  			goto retry;
>  		}
>  	}
> +
> +	mm_fault_accounting(tsk, regs, address, major);
> +
>  	if (IS_ENABLED(CONFIG_PGSTE) && gmap) {
>  		address =  __gmap_link(gmap, current->thread.gmap_addr,
>  				       address);
> -- 
> 2.26.2
>