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

[Peter Xu <peterx@redhat.com>]

On Tue, Sep 24, 2019 at 08:45:18AM -0700, Matthew Wilcox wrote:
> On Tue, Sep 24, 2019 at 11:19:08AM +0800, Peter Xu wrote:
> > 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.
> 
> I think the way to do this is to introduce fault_signal_pending(),
> converting the architectures to it that match that pattern.  Then one
> patch per architecture to convert the ones which use a different pattern
> to the same pattern.

Fair enough.  I can start with a fault_signal_pending() only keeps the
sigkill handling just like before, then convert all the archs, with
the last patch to only touch fault_signal_pending() for non-fatal
signals.

> 
> Oh, and while you're looking at the callers of handle_mm_fault(), a
> lot of them don't check conditions in the right order.  x86, at least,
> handles FAULT_RETRY before handling FAULT_ERROR, which is clearly wrong.
> 
> Kirill and I recently discussed it here:
> https://lore.kernel.org/linux-mm/20190911152338.gqqgxrmqycodfocb@box/T/

Hmm sure.  These sound very reasonable.

I must admit that I am not brave enough to continue grow my patchset
on my own.  The condition I'm facing right now is that I can't really
find enough reviewers for this series (Linus helped me quite a lot, I
really, really, appreciated that), while it's still growing.  I hope
the started discussion means that you'll be at least another potential
reviewer (oh, should I count Kirill in as well? :) at least to the
coming patches for the things mentioned above.

Thanks,

-- 
Peter Xu