Re: [PATCH v2 1/1] mm/mmu_gather: replace IPI with synchronize_rcu() when batch allocation fails

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Feb 24, 2026 at 11:07:00AM +0800, Lance Yang wrote:
> From: Lance Yang <lance.yang@linux.dev>
> 
> When freeing page tables, we try to batch them. If batch allocation fails
> (GFP_NOWAIT), __tlb_remove_table_one() immediately frees the one without
> batching.
> 
> On !CONFIG_PT_RECLAIM, the fallback sends an IPI to all CPUs via
> tlb_remove_table_sync_one(). It disrupts all CPUs even when only a single
> process is unmapping memory. IPI broadcast was reported to hurt RT
> workloads[1].
> 
> tlb_remove_table_sync_one() synchronizes with lockless page-table walkers
> (e.g. GUP-fast) that rely on IRQ disabling. These walkers use
> local_irq_disable(), which is also an RCU read-side critical section.
> 
> This patch introduces tlb_remove_table_sync_rcu() which uses RCU grace
> period (synchronize_rcu()) instead of IPI broadcast. This provides the
> same guarantee as IPI but without disrupting all CPUs. Since batch
> allocation already failed, we are in a way slow path where sleeping is
> acceptable - we are in process context (unmap_region, exit_mmap) with only
> mmap_lock held. might_sleep() will catch any invalid context.

So sending the IPIs also requires non-atomic context, so change there.

What isn't explained, and very much not clear to me, is why
tlb_remove_table_sync_one() is retained?


> diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
> index 4aeac0c3d3f0..bdcc2778ac64 100644
> --- a/include/asm-generic/tlb.h
> +++ b/include/asm-generic/tlb.h
> @@ -251,6 +251,8 @@ static inline void tlb_remove_table(struct mmu_gather *tlb, void *table)
>  
>  void tlb_remove_table_sync_one(void);
>  
> +void tlb_remove_table_sync_rcu(void);
> +
>  #else
>  
>  #ifdef tlb_needs_table_invalidate
> @@ -259,6 +261,8 @@ void tlb_remove_table_sync_one(void);
>  
>  static inline void tlb_remove_table_sync_one(void) { }
>  
> +static inline void tlb_remove_table_sync_rcu(void) { }
> +
>  #endif /* CONFIG_MMU_GATHER_RCU_TABLE_FREE */
>  
>  
> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
> index fe5b6a031717..2c6fa8db55df 100644
> --- a/mm/mmu_gather.c
> +++ b/mm/mmu_gather.c
> @@ -296,6 +296,26 @@ static void tlb_remove_table_free(struct mmu_table_batch *batch)
>  	call_rcu(&batch->rcu, tlb_remove_table_rcu);
>  }
>  
> +/**
> + * tlb_remove_table_sync_rcu() - synchronize with software page-table walkers
> + *
> + * Like tlb_remove_table_sync_one() but uses RCU grace period instead of IPI
> + * broadcast. Use in slow paths where sleeping is acceptable.
> + *
> + * Software/Lockless page-table walkers use local_irq_disable(), which is also
> + * an RCU read-side critical section. synchronize_rcu() waits for all such
> + * sections, providing the same guarantee as tlb_remove_table_sync_one() but
> + * without disrupting all CPUs with IPIs.
> + *
> + * Do not use for freeing memory. Use RCU callbacks instead to avoid latency
> + * spikes. Cannot be called from any atomic context.
> + */
> +void tlb_remove_table_sync_rcu(void)
> +{
> +	might_sleep();
> +	synchronize_rcu();

synchronize_rcu() should end up in a might_sleep() at some point if it
blocks (which it typically will).

> +}
> +
>  #else /* !CONFIG_MMU_GATHER_RCU_TABLE_FREE */
>  
>  static void tlb_remove_table_free(struct mmu_table_batch *batch)
> @@ -339,7 +359,7 @@ static inline void __tlb_remove_table_one(void *table)
>  #else
>  static inline void __tlb_remove_table_one(void *table)
>  {
> -	tlb_remove_table_sync_one();
> +	tlb_remove_table_sync_rcu();
>  	__tlb_remove_table(table);
>  }
>  #endif /* CONFIG_PT_RECLAIM */
> -- 
> 2.49.0
>