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

Peter Xu
2022-05-30T16:02:30Z

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:
> >
> >
> > Am 29.05.22 um 22:33 schrieb Heiko Carstens:
> > [...]
> > >
> > > Guess the patch below on top of your patch is what we want.
> > > Just for clarification: if gmap is not NULL then the process is a kvm
> > > process. So, depending on the workload, this optimization makes sense.
> > >
> > > 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;
> > > + }
> > > + 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,

--
Peter Xu