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

Heiko Carstens
2022-05-30T17:06:54Z

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.