Re: [PATCH v5 08/12] mm: enable lazy_mmu sections to nest

["David Hildenbrand (Red Hat)" <david@xxxxxxxxxx>]

Some comments from my side:


> >   static inline void arch_enter_lazy_mmu_mode(void)
> >   {
> > -       /*
> > -        * lazy_mmu_mode is not supposed to permit nesting. But in practice this
> > -        * does happen with CONFIG_DEBUG_PAGEALLOC, where a page allocation
> > -        * inside a lazy_mmu_mode section (such as zap_pte_range()) will change
> > -        * permissions on the linear map with apply_to_page_range(), which
> > -        * re-enters lazy_mmu_mode. So we tolerate nesting in our
> > -        * implementation. The first call to arch_leave_lazy_mmu_mode() will
> > -        * flush and clear the flag such that the remainder of the work in the
> > -        * outer nest behaves as if outside of lazy mmu mode. This is safe and
> > -        * keeps tracking simple.
> > -        */
> > -
> >         set_thread_flag(TIF_LAZY_MMU);>  }
> Should not platform specific changes be deferred to subsequent patches until
> nesting is completely enabled in generic first ? Although no problem as such
> but would be bit cleaner.
This could indeed be done in a separate patch. But I also don't see a 
problem with updating the doc in this patch.
> >   
> > diff --git a/include/linux/mm_types_task.h b/include/linux/mm_types_task.h
> > index a82aa80c0ba4..11bf319d78ec 100644
> > --- a/include/linux/mm_types_task.h
> > +++ b/include/linux/mm_types_task.h
> > @@ -88,4 +88,9 @@ struct tlbflush_unmap_batch {
> >   #endif
> >   };
> >   
> > +struct lazy_mmu_state {
> > +       u8 enable_count;
> > +       u8 pause_count;
> > +};
> > +
> Should not this be wrapped with CONFIG_ARCH_HAS_LAZY_MMU_MODE as the task_struct
> element 'lazy_mmu_state' is only available with the feature.
No strong opinion; the compiler will ignore it either way. And less 
ifdef is good, right? :)
... and there is nothing magical in there that would result in other 
dependencies.
> Besides, is a depth
> of 256 really expected here ? 4 bits for each element would not be sufficient 
> for
> a depth of 16 ?

We could indeed use something like

struct lazy_mmu_state {
        u8 enable_count : 4;
        u8 pause_count : 4;
};

but then, the individual operations on enable_count/pause_count need 
more instructions.
Further, as discussed, this 1 additional byte barely matters given the 
existing size of the task struct.
No strong opinion.

> >    */
> >   #ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE
> > +/**
> > + * lazy_mmu_mode_enable() - Enable the lazy MMU mode.
> > + *
> > + * Enters a new lazy MMU mode section; if the mode was not already enabled,
> > + * enables it and calls arch_enter_lazy_mmu_mode().
> > + *
> > + * Must be paired with a call to lazy_mmu_mode_disable().
> > + *
> > + * Has no effect if called:
> > + * - While paused - see lazy_mmu_mode_pause()
> > + * - In interrupt context
> > + */
> >   static inline void lazy_mmu_mode_enable(void)
> >   {
> > -       if (in_interrupt())
> > +       struct lazy_mmu_state *state = &current->lazy_mmu_state;
> > +
> > +       if (in_interrupt() || state->pause_count > 0)
> >                 return;
> >   
> > -	arch_enter_lazy_mmu_mode();
> > +       VM_WARN_ON_ONCE(state->enable_count == U8_MAX);
> > +
> > +       if (state->enable_count++ == 0)
> > +               arch_enter_lazy_mmu_mode();
> When lazy_mmu_mode_enable() gets called for the first time with 
> state->enable_count as 0,
> then arch_enter_lazy_mmu_mode() will not get called ? Bit confused.

state->enable_count++ returns the old value (0). Are you thinking of
++state->enable_count?

But maybe I misudnerstood your concern.

[...]

> > +/**
> > + * lazy_mmu_mode_pause() - Resume the lazy MMU mode.
> > + *
> > + * Resumes the lazy MMU mode; if it was active at the point where the matching
> > + * call to lazy_mmu_mode_pause() was made, re-enables it and calls
> > + * arch_enter_lazy_mmu_mode().
> > + *
> > + * Must match a call to lazy_mmu_mode_pause().
> > + *
> > + * Has no effect if called:
> > + * - While paused (inside another pause()/resume() pair)
> > + * - In interrupt context
> > + */
> >   static inline void lazy_mmu_mode_resume(void)
> >   {
> > +       struct lazy_mmu_state *state = &current->lazy_mmu_state;
> > +
> >         if (in_interrupt())
> >                 return;
> >   
> > -	arch_enter_lazy_mmu_mode();
> > +       VM_WARN_ON_ONCE(state->pause_count == 0);
> > +
> > +       if (--state->pause_count == 0 && state->enable_count > 0)
> > +               arch_enter_lazy_mmu_mode();
> >   }
> Should not state->pause/enable_count tests and increment/decrement be handled
> inside include/linux/sched via helpers like in_lazy_mmu_mode() ? This is will
> ensure cleaner abstraction with respect to task_struct.
I don't think this is required given that this code here implements
CONFIG_ARCH_HAS_LAZY_MMU_MODE support.

--
Cheers

David