linux-kernel - Re: [PATCH 18/26] mm/s390: Use general page fault accounting

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Date:   Wed, 24 Jun 2020 16:40:47 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Gerald Schaefer <gerald.schaefer@...ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Will Deacon <will@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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 18/26] mm/s390: Use general page fault accounting

On Wed, Jun 24, 2020 at 08:49:30PM +0200, Gerald Schaefer wrote:
> On Fri, 19 Jun 2020 12:13:35 -0400
> Peter Xu <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: 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 | 16 +---------------
> >  1 file changed, 1 insertion(+), 15 deletions(-)
> > 
> > diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> > index ab6d7eedcfab..4d62ca7d3e09 100644
> > --- a/arch/s390/mm/fault.c
> > +++ b/arch/s390/mm/fault.c
> > @@ -479,7 +479,7 @@ 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)
> > @@ -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;
> 
> There are two cases here where we skipped the accounting,
> fault_signal_pending() and VM_FAULT_ERROR, similar to other archs.
> 
> fault_signal_pending() should be ok, because that only seems to be true
> for fault & VM_FAULT_RETRY, in which case the new approach also skips
> the accounting.

IMHO it's still possible to have fault_signal_pending() return true even if the
fault is not with VM_FAULT_RETRY, e.g., when the signal is delivered right
after the fault is correctly handled for the thread.  However I hope we can
avoid considering that too even if so...

> 
> But for VM_FAULT_ERROR, the new approach would do accounting, IIUC. Is
> that changed on purpose? See also my reply on [PATCH 01/26].

(replied in the other thread)

Thanks,

-- 
Peter Xu