Subject: Re: [PATCH v4 01/12] powerpc/64s: Do not re-activate batched TLB flush

From: Ritesh Harjani (IBM) <[email-masked]>
Date: Wed, 05 Nov 2025 08:16:58 +0530

Kevin Brodsky <[email-masked]> writes:

> From: Alexander Gordeev <[email-masked]>
>
> Since commit b9ef323ea168 ("powerpc/64s: Disable preemption in hash
> lazy mmu mode") a task can not be preempted while in lazy MMU mode.
> Therefore, the batch re-activation code is never called, so remove it.
>
> Signed-off-by: Alexander Gordeev <[email-masked]>
> Signed-off-by: Kevin Brodsky <[email-masked]>
> ---
>  arch/powerpc/include/asm/thread_info.h |  2 --
>  arch/powerpc/kernel/process.c          | 25 -------------------------
>  2 files changed, 27 deletions(-)
>

Since the commit referenced in above disables the preemption in
arch_enter_lazy_mmu(), so the expectation is that we will never be
context switched while in lazy_mmu, hence the code changes in
switch_to() around __flush_tlb_pending() should ideally never be called.

With this analysis - the patch looks good to me. I will give this entire
patch series a try on Power HW with Hash mmu too (which uses lazy mmu and
let you know the results of that)!

For this patch please feel free to add:
Reviewed-by: Ritesh Harjani (IBM) <[email-masked]>

CC: Venkat who also runs CI on linux Power HW for upstream testing :)

-ritesh

> diff --git a/arch/powerpc/include/asm/thread_info.h b/arch/powerpc/include/asm/thread_info.h
> index b0f200aba2b3..97f35f9b1a96 100644
> --- a/arch/powerpc/include/asm/thread_info.h
> +++ b/arch/powerpc/include/asm/thread_info.h
> @@ -154,12 +154,10 @@ void arch_setup_new_exec(void);
>  /* Don't move TLF_NAPPING without adjusting the code in entry_32.S */
>  #define TLF_NAPPING		0	/* idle thread enabled NAP mode */
>  #define TLF_SLEEPING		1	/* suspend code enabled SLEEP mode */
> -#define TLF_LAZY_MMU		3	/* tlb_batch is active */
>  #define TLF_RUNLATCH		4	/* Is the runlatch enabled? */
>  
>  #define _TLF_NAPPING		(1 << TLF_NAPPING)
>  #define _TLF_SLEEPING		(1 << TLF_SLEEPING)
> -#define _TLF_LAZY_MMU		(1 << TLF_LAZY_MMU)
>  #define _TLF_RUNLATCH		(1 << TLF_RUNLATCH)
>  
>  #ifndef __ASSEMBLER__
> diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> index eb23966ac0a9..9237dcbeee4a 100644
> --- a/arch/powerpc/kernel/process.c
> +++ b/arch/powerpc/kernel/process.c
> @@ -1281,9 +1281,6 @@ struct task_struct *__switch_to(struct task_struct *prev,
>  {
>  	struct thread_struct *new_thread, *old_thread;
>  	struct task_struct *last;
> -#ifdef CONFIG_PPC_64S_HASH_MMU
> -	struct ppc64_tlb_batch *batch;
> -#endif
>  
>  	new_thread = &new->thread;
>  	old_thread = &current->thread;
> @@ -1291,14 +1288,6 @@ struct task_struct *__switch_to(struct task_struct *prev,
>  	WARN_ON(!irqs_disabled());
>  
>  #ifdef CONFIG_PPC_64S_HASH_MMU
> -	batch = this_cpu_ptr(&ppc64_tlb_batch);
> -	if (batch->active) {
> -		current_thread_info()->local_flags |= _TLF_LAZY_MMU;
> -		if (batch->index)
> -			__flush_tlb_pending(batch);
> -		batch->active = 0;
> -	}
> -
>  	/*
>  	 * On POWER9 the copy-paste buffer can only paste into
>  	 * foreign real addresses, so unprivileged processes can not
> @@ -1369,20 +1358,6 @@ struct task_struct *__switch_to(struct task_struct *prev,
>  	 */
>  
>  #ifdef CONFIG_PPC_BOOK3S_64
> -#ifdef CONFIG_PPC_64S_HASH_MMU
> -	/*
> -	 * This applies to a process that was context switched while inside
> -	 * arch_enter_lazy_mmu_mode(), to re-activate the batch that was
> -	 * deactivated above, before _switch(). This will never be the case
> -	 * for new tasks.
> -	 */
> -	if (current_thread_info()->local_flags & _TLF_LAZY_MMU) {
> -		current_thread_info()->local_flags &= ~_TLF_LAZY_MMU;
> -		batch = this_cpu_ptr(&ppc64_tlb_batch);
> -		batch->active = 1;
> -	}
> -#endif
> -
>  	/*
>  	 * Math facilities are masked out of the child MSR in copy_thread.
>  	 * A new task does not need to restore_math because it will
> -- 
> 2.47.0