From mboxrd@z Thu Jan  1 00:00:00 1970
From: Heiko Carstens <hca@linux.ibm.com>
Subject: Re: [PATCH v4] mm: Avoid unnecessary page fault retires on shared
 memory types
Date: Mon, 30 May 2022 19:03:31 +0200
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To: Peter Xu <peterx@redhat.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Will Deacon <will@kernel.org>, Matt Turner <mattst88@gmail.com>, linux-s390@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>, Brian Cain <bcain@quicinc.com>, Borislav Petkov <bp@alien8.de>, linux-alpha@vger.kernel.org, Alistair Popple <apopple@nvidia.com>, Jonas Bonn <jonas@southpole.se>, linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org, Michael Ellerman <mpe@ellerman.id.au>, Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>, linux-snps-arc@lists.infradead.org, Vineet Gupta <vgupta@kernel.org>, Vasily Gorbik <gor@linux.ibm.com>, Vlastimil Babka <vbabka@suse.cz>, Ivan Kokshaysky <ink@jurassic.park.msu.ru>, Rich Felker <dalias@li>

On Mon, May 30, 2022 at 12:00:52PM -0400, Peter Xu wrote:
> On Mon, May 30, 2022 at 11:52:54AM -0400, Peter Xu wrote:
> > On Mon, May 30, 2022 at 11:35:10AM +0200, Christian Borntraeger wrote:
> > > > diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> > > > index 4608cc962ecf..e1d40ca341b7 100644
> > > > --- a/arch/s390/mm/fault.c
> > > > +++ b/arch/s390/mm/fault.c
> > > > @@ -436,12 +436,11 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
> > > >   	/* The fault is fully completed (including releasing mmap lock) */
> > > >   	if (fault & VM_FAULT_COMPLETED) {
> > > > -		/*
> > > > -		 * Gmap will need the mmap lock again, so retake it.  TODO:
> > > > -		 * only conditionally take the lock when CONFIG_PGSTE set.
> > > > -		 */
> > > > -		mmap_read_lock(mm);
> > > > -		goto out_gmap;
> > > > +		if (gmap) {
> > > > +			mmap_read_lock(mm);
> > > > +			goto out_gmap;
> > > > +		}
			fault = 0;  <----
> > > > +		goto out;
> 
> Hmm, right after I replied I found "goto out" could be problematic, since
> all s390 callers of do_exception() will assume it an error condition (side
> note: "goto out_gmap" contains one step to clear "fault" to 0).  I'll
> replace this with "return 0" instead if it looks good to both of you.
> 
> I'll wait for a confirmation before reposting.  Thanks,

Right, that was stupid. Thanks for double checking!

However could you please add "fault = 0" just in front of the goto out
like above? I'd like to avoid having returns and gotos mixed.