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

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