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

[Alexander Gordeev <agordeev@linux.ibm.com>]

On Mon, Nov 24, 2025 at 01:22:24PM +0000, Kevin Brodsky wrote:
...
> + * Nesting is permitted: <code> may itself use an enable()/disable() pair.
> + * A nested call to enable() has no functional effect; however disable() causes
> + * any batched architectural state to be flushed regardless of nesting. After a
> + * call to disable(), the caller can therefore rely on all previous page table
> + * modifications to have taken effect, but the lazy MMU mode may still be
> + * enabled.
> + *
> + * In certain cases, it may be desirable to temporarily pause the lazy MMU mode.
> + * This can be done using:
> + *
> + *   lazy_mmu_mode_pause();
> + *   <code>
> + *   lazy_mmu_mode_resume();
> + *
> + * pause() ensures that the mode is exited regardless of the nesting level;
> + * resume() re-enters the mode at the same nesting level. Any call to the
> + * lazy_mmu_mode_* API between those two calls has no effect. In particular,
> + * this means that pause()/resume() pairs may nest.
> + *
> + * in_lazy_mmu_mode() can be used to check whether the lazy MMU mode is
> + * currently enabled.

The in_lazy_mmu_mode() name looks ambiguous to me. When the lazy MMU mode
is paused are we still in lazy MMU mode? The __task_lazy_mmu_mode_active()
implementation suggests we are not, while one could still assume we are,
just paused.

Should in_lazy_mmu_mode() be named e.g. as in_active_lazy_mmu_mode() such
a confusion would not occur in the first place.

>   */
...
> +#ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE
> +/**
> + * __task_lazy_mmu_mode_active() - Test the lazy MMU mode state for a task.
> + * @tsk: The task to check.
> + *
> + * Test whether @tsk has its lazy MMU mode state set to active (i.e. enabled
> + * and not paused).
> + *
> + * This function only considers the state saved in task_struct; to test whether
> + * current actually is in lazy MMU mode, in_lazy_mmu_mode() should be used
> + * instead.
> + *
> + * This function is intended for architectures that implement the lazy MMU
> + * mode; it must not be called from generic code.
> + */
> +static inline bool __task_lazy_mmu_mode_active(struct task_struct *tsk)
> +{
> +	struct lazy_mmu_state *state = &tsk->lazy_mmu_state;
> +
> +	return state->enable_count > 0 && state->pause_count == 0;
> +}
> +
> +/**
> + * in_lazy_mmu_mode() - Test whether we are currently in lazy MMU mode.
> + *
> + * Test whether the current context is in lazy MMU mode. This is true if both:
> + * 1. We are not in interrupt context
> + * 2. Lazy MMU mode is active for the current task
> + *
> + * This function is intended for architectures that implement the lazy MMU
> + * mode; it must not be called from generic code.
> + */
> +static inline bool in_lazy_mmu_mode(void)
> +{
> +	if (in_interrupt())
> +		return false;
> +
> +	return __task_lazy_mmu_mode_active(current);
> +}
> +#endif
...