Re: [PATCH v4 05/10] mm: Return faster for non-fatal signals in user mode faults

[Peter Xu <peterx@redhat.com>]

On Mon, Sep 23, 2019 at 07:54:47PM -0700, Matthew Wilcox wrote:
> On Tue, Sep 24, 2019 at 10:47:21AM +0800, Peter Xu wrote:
> > On Mon, Sep 23, 2019 at 11:03:49AM -0700, Linus Torvalds wrote:
> > > On Sun, Sep 22, 2019 at 9:26 PM Peter Xu <peterx@redhat.com> wrote:
> > > >
> > > > This patch is a preparation of removing that special path by allowing
> > > > the page fault to return even faster if we were interrupted by a
> > > > non-fatal signal during a user-mode page fault handling routine.
> > > 
> > > So I really wish saome other vm person would also review these things,
> > > but looking over this series once more, this is the patch I probably
> > > like the least.
> > > 
> > > And the reason I like it the least is that I have a hard time
> > > explaining to myself what the code does and why, and why it's so full
> > > of this pattern:
> > > 
> > > > -       if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
> > > > +       if ((fault & VM_FAULT_RETRY) &&
> > > > +           fault_should_check_signal(user_mode(regs)))
> > > >                 return;
> > > 
> > > which isn't all that pretty.
> > > 
> > > Why isn't this just
> > > 
> > >   static bool fault_signal_pending(unsigned int fault_flags, struct
> > > pt_regs *regs)
> > >   {
> > >         return (fault_flags & VM_FAULT_RETRY) &&
> > >                 (fatal_signal_pending(current) ||
> > >                  (user_mode(regs) && signal_pending(current)));
> > >   }
> > > 
> > > and then most of the users would be something like
> > > 
> > >         if (fault_signal_pending(fault, regs))
> > >                 return;
> > > 
> > > and the exceptions could do their own thing.
> > > 
> > > Now the code is prettier and more understandable, I feel.
> > > 
> > > And if something doesn't follow this pattern, maybe it either _should_
> > > follow that pattern or it should just not use the helper but explain
> > > why it has an unusual pattern.
> 
> > +++ b/arch/alpha/mm/fault.c
> > @@ -150,7 +150,7 @@ do_page_fault(unsigned long address, unsigned long mmcsr,
> >  	   the fault.  */
> >  	fault = handle_mm_fault(vma, address, flags);
> >  
> > -	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
> > +	if (fault_signal_pending(fault, regs))
> >  		return;
> >  
> >  	if (unlikely(fault & VM_FAULT_ERROR)) {
> 
> > +++ b/arch/arm/mm/fault.c
> > @@ -301,6 +301,11 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
> >  		return 0;
> >  	}
> >  
> > +	/* Fast path to handle user mode signals */
> > +	if ((fault & VM_FAULT_RETRY) && user_mode(regs) &&
> > +	    signal_pending(current))
> > +		return 0;
> 
> But _why_ are they different?  This is a good opportunity to make more
> code the same between architectures.

(Thanks for joining the discussion)

I'd like to do these - my only worry is that I can't really test them
well simply because I don't have all the hardwares.  For now the
changes are mostly straightforward so I'm relatively confident (not to
mention the code needs proper reviews too, and of course I would
appreciate much if anyone wants to smoke test it).  If I change it in
a drastic way, I won't be that confident without some tests at least
on multiple archs (not to mention that even smoke testing across major
archs will be a huge amount of work...).  So IMHO those might be more
suitable as follow-up for per-arch developers if we can at least reach
a consensus on the whole idea of this patchset.

Thanks,

-- 
Peter Xu