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

From: Ritesh Harjani (IBM) <[email-masked]>
Date: Wed, 05 Nov 2025 14:19:03 +0530

Kevin Brodsky <[email-masked]> writes:

> Despite recent efforts to prevent lazy_mmu sections from nesting, it
> remains difficult to ensure that it never occurs - and in fact it
> does occur on arm64 in certain situations (CONFIG_DEBUG_PAGEALLOC).
> Commit 1ef3095b1405 ("arm64/mm: Permit lazy_mmu_mode to be nested")
> made nesting tolerable on arm64, but without truly supporting it:
> the inner call to leave() disables the batching optimisation before
> the outer section ends.
>
> This patch actually enables lazy_mmu sections to nest by tracking
> the nesting level in task_struct, in a similar fashion to e.g.
> pagefault_{enable,disable}(). This is fully handled by the generic
> lazy_mmu helpers that were recently introduced.
>
> lazy_mmu sections were not initially intended to nest, so we need to
> clarify the semantics w.r.t. the arch_*_lazy_mmu_mode() callbacks.
> This patch takes the following approach:
>
> * The outermost calls to lazy_mmu_mode_{enable,disable}() trigger
>   calls to arch_{enter,leave}_lazy_mmu_mode() - this is unchanged.
>
> * Nested calls to lazy_mmu_mode_{enable,disable}() are not forwarded
>   to the arch via arch_{enter,leave} - lazy MMU remains enabled so
>   the assumption is that these callbacks are not relevant. However,
>   existing code may rely on a call to disable() to flush any batched
>   state, regardless of nesting. arch_flush_lazy_mmu_mode() is
>   therefore called in that situation.
>
> A separate interface was recently introduced to temporarily pause
> the lazy MMU mode: lazy_mmu_mode_{pause,resume}(). pause() fully
> exits the mode *regardless of the nesting level*, and resume()
> restores the mode at the same nesting level.
>
> Whether the mode is actually enabled or not at any point is tracked
> by a separate "active" field in task_struct; this makes it possible
> to check invariants in the generic API, and to expose a new
> in_lazy_mmu_mode() helper to replace the various ways arch's
> currently track whether the mode is enabled (this will be done in
> later patches).
>
> In summary (nesting/active represent the values *after* the call):
>
> lazy_mmu_mode_enable()		-> arch_enter()	    nesting=1 active=1
>     lazy_mmu_mode_enable()	-> ø		    nesting=2 active=1
> 	lazy_mmu_mode_pause()	-> arch_leave()     nesting=2 active=0
> 	lazy_mmu_mode_resume()	-> arch_enter()     nesting=2 active=1
>     lazy_mmu_mode_disable()	-> arch_flush()     nesting=1 active=1
> lazy_mmu_mode_disable()		-> arch_leave()     nesting=0 active=0
>
> Note: in_lazy_mmu_mode() is added to <linux/sched.h> to allow arch
> headers included by <linux/pgtable.h> to use it.
>
> Signed-off-by: Kevin Brodsky <[email-masked]>
> ---
>  arch/arm64/include/asm/pgtable.h | 12 ------
>  include/linux/mm_types_task.h    |  5 +++
>  include/linux/pgtable.h          | 67 ++++++++++++++++++++++++++++++--
>  include/linux/sched.h            | 16 ++++++++
>  4 files changed, 84 insertions(+), 16 deletions(-)
>
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 54f8d6bb6f22..535435248923 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -82,18 +82,6 @@ static inline void queue_pte_barriers(void)
>  
>  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.
> -	 */
> -
>  	if (in_interrupt())
>  		return;
>  
> diff --git a/include/linux/mm_types_task.h b/include/linux/mm_types_task.h
> index a82aa80c0ba4..632d404f8191 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 nesting_level;
> +	bool active;
> +};
> +
>  #endif /* _LINUX_MM_TYPES_TASK_H */
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index b5fdf32c437f..e6064e00b22d 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -228,27 +228,86 @@ static inline int pmd_dirty(pmd_t pmd)
>   * of the lazy mode. So the implementation must assume preemption may be enabled
>   * and cpu migration is possible; it must take steps to be robust against this.
>   * (In practice, for user PTE updates, the appropriate page table lock(s) are
> - * held, but for kernel PTE updates, no lock is held). Nesting is not permitted
> - * and the mode cannot be used in interrupt context.
> + * held, but for kernel PTE updates, no lock is held). The mode cannot be used
> + * in interrupt context.
> + *
> + * The lazy MMU mode is enabled for a given block of code using:
> + *
> + *   lazy_mmu_mode_enable();
> + *   <code>
> + *   lazy_mmu_mode_disable();
> + *
> + * 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();
> + *
> + * This sequence must only be used if the lazy MMU mode is already enabled.
> + * pause() ensures that the mode is exited regardless of the nesting level;
> + * resume() re-enters the mode at the same nesting level. <code> must not modify
> + * the lazy MMU state (i.e. it must not call any of the lazy_mmu_mode_*
> + * helpers).
> + *
> + * 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.

  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) {
 +		arch_enter_lazy_mmu_mode();
 +		state->active = true;
 +	}
  }

... I think it make more sense to enable the state after the arch_**
call right.

>  
>  static inline void lazy_mmu_mode_disable(void)
>  {
> -	arch_leave_lazy_mmu_mode();
> +	struct lazy_mmu_state *state = &current->lazy_mmu_state;
> +
> +	VM_WARN_ON_ONCE(state->nesting_level == 0);
> +	VM_WARN_ON(!state->active);
> +
> +	if (--state->nesting_level == 0) {
> +		state->active = false;
> +		arch_leave_lazy_mmu_mode();
> +	} else {
> +		/* Exiting a nested section */
> +		arch_flush_lazy_mmu_mode();
> +	}
>  }

This looks ok though.

>  
>  static inline void lazy_mmu_mode_pause(void)
>  {
> +	struct lazy_mmu_state *state = &current->lazy_mmu_state;
> +
> +	VM_WARN_ON(state->nesting_level == 0 || !state->active);
> +
> +	state->active = false;
>  	arch_leave_lazy_mmu_mode();
>  }
>  
>  static inline void lazy_mmu_mode_resume(void)
>  {
> +	struct lazy_mmu_state *state = &current->lazy_mmu_state;
> +
> +	VM_WARN_ON(state->nesting_level == 0 || state->active);
> +
> +	state->active = true;
>  	arch_enter_lazy_mmu_mode();
>  }

Ditto.

-ritesh

>  #else
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index cbb7340c5866..11566d973f42 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1441,6 +1441,10 @@ struct task_struct {
>  
>  	struct page_frag		task_frag;
>  
> +#ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE
> +	struct lazy_mmu_state		lazy_mmu_state;
> +#endif
> +
>  #ifdef CONFIG_TASK_DELAY_ACCT
>  	struct task_delay_info		*delays;
>  #endif
> @@ -1724,6 +1728,18 @@ static inline char task_state_to_char(struct task_struct *tsk)
>  	return task_index_to_char(task_state_index(tsk));
>  }
>  
> +#ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE
> +static inline bool in_lazy_mmu_mode(void)
> +{
> +	return current->lazy_mmu_state.active;
> +}
> +#else
> +static inline bool in_lazy_mmu_mode(void)
> +{
> +	return false;
> +}
> +#endif
> +
>  extern struct pid *cad_pid;
>  
>  /*
> -- 
> 2.47.0