[PATCH v4] mm: Avoid unnecessary page fault retires on shared memory types

[Heiko Carstens]

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.