Re: [PATCH v4 07/12] mm: enable lazy_mmu sections to nest

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

Alexander Gordeev <agordeev@linux.ibm.com> writes:

> On Wed, Nov 05, 2025 at 02:19:03PM +0530, Ritesh Harjani wrote:
>> > + * in_lazy_mmu_mode() can be used to check whether the lazy MMU mode is
>> > + * currently enabled.
>> >   */
>> >  #ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE
>> >  static inline void lazy_mmu_mode_enable(void)
>> >  {
>> > -	arch_enter_lazy_mmu_mode();
>> > +	struct lazy_mmu_state *state = &current->lazy_mmu_state;
>> > +
>> > +	VM_WARN_ON_ONCE(state->nesting_level == U8_MAX);
>> > +	/* enable() must not be called while paused */
>> > +	VM_WARN_ON(state->nesting_level > 0 && !state->active);
>> > +
>> > +	if (state->nesting_level++ == 0) {
>> > +		state->active = true;
>> > +		arch_enter_lazy_mmu_mode();
>> > +	}
>> >  }
>> 
>> Some architectures disables preemption in their
>> arch_enter_lazy_mmu_mode(). So shouldn't the state->active = true should
>> happen after arch_enter_lazy_mmu_mode() has disabled preemption()? i.e.
>
> Do you have some scenario in mind that could cause an issue?
>
No not really. But that's a deviation from what previous arch hooks were
expecting. Although thinking this through - I don't have any usecase
where this can be a problem. 

But let me re-visit some of the code paths on ppc64 lazy mmu... 

Looking at the arch specific usecase I see we always do get_cpu_var()
for accessing the per-cpu batch array which disables preemption before
accessing the per-cpu structure.. This per-cpu structure is where we
batch pte updates... 

For e.g... 
  
    arch_enter_lazy_mmu_mode()
        hpte_need_flush()
            get_cpu_var()   // this takes care of preempt_disable() 
            adds vpns to per-cpu batch[i]
            put_cpu_var()   // 
    arch_leave_lazy_mmu_mode()

> IOW, what could go wrong if the process is scheduled to another
> CPU before preempt_disable() is called?

So from above - I don't think your sequence to update
   state->active = true 
before calling arch_enter hook should be a problem.
Based on above this looks mostly ok to me.

-ritesh