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

[Matthew Wilcox]

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@xxxxxxxxxx> 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.