Re: [PATCH v3 07/13] mm: enable lazy_mmu sections to nest

[David Hildenbrand <david@xxxxxxxxxx>]

> > > + * currently enabled.
> > >     */
> > >    #ifdef CONFIG_ARCH_LAZY_MMU
> > >    static inline void lazy_mmu_mode_enable(void)
> > >    {
> > > -    arch_enter_lazy_mmu_mode();
> > > +    struct lazy_mmu_state *state = &current->lazy_mmu_state;
> > > +
> > > +    VM_BUG_ON(state->count == U8_MAX);
> > No VM_BUG_ON() please.
> I did wonder if this would be acceptable!
Use VM_WARN_ON_ONCE() and let early testing find any such issues.

VM_* is active in debug kernels only either way! :)

If you'd want to handle this in production kernels you'd need

if (WARN_ON_ONCE()) {
        /* Try to recover */
}

And that seems unnecessary/overly-complicated for something that should 
never happen, and if it happens, can be found early during testing.
> What should we do in case of underflow/overflow then? Saturate or just
> let it wrap around? If an overflow occurs we're probably in some
> infinite recursion and we'll crash anyway, but an underflow is likely
> due to a double disable() and saturating would probably allow to recover.
>
> > > +    /* enable() must not be called while paused */
> > > +    VM_WARN_ON(state->count > 0 && !state->enabled);
> > > +
> > > +    if (state->count == 0) {
> > > +        arch_enter_lazy_mmu_mode();
> > > +        state->enabled = true;
> > > +    }
> > > +    ++state->count;
> > Can do
> >
> > if (state->count++ == 0) {
> My idea here was to have exactly the reverse order between enable() and
> disable(), so that arch_enter() is called before lazy_mmu_state is
> updated, and arch_leave() afterwards. arch_* probably shouldn't rely on
> this (or care), but I liked the symmetry.
I see, but really the arch callback should never have to care about that
value -- unless something is messed up :)

[...]

> > > +static inline bool in_lazy_mmu_mode(void)
> > So these functions will reveal the actual arch state, not whether
> > _enabled() was called.
> >
> > As I can see in later patches, in interrupt context they are also
> > return "not in lazy mmu mode".
> Yes - the idea is that a task is in lazy MMU mode if it enabled it and
> is in process context. The mode is never enabled in interrupt context.
> This has always been the intention, but it wasn't formalised until patch
> 12 (except on arm64).
Okay, thanks for clarifying.

--
Cheers

David / dhildenb