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

[Kevin Brodsky <kevin.brodsky@xxxxxxx>]

On 06/11/2025 16:32, Ritesh Harjani (IBM) wrote:
> Alexander Gordeev <agordeev@xxxxxxxxxxxxx> 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.

Which arch hook expectations are you referring to?

> 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...

arch_enter() disables preemption so accesses to per-CPU variables
anywhere in the section shouldn't be an issue either way.

The bigger picture (regarding patch 9) is that what in_lazy_mmu_state()
returns is based on the current task's state (not a per-CPU variable),
and always false while in interrupt. As a result whether preemption is
disabled or not should make no difference, only program order matters.

- Kevin

> 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