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

[David Hildenbrand (Arm)]

On 2/24/26 04:07, Lance Yang wrote:
> From: Lance Yang <lance.yang@xxxxxxxxx>
> 
> 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.
> 
> [1] https://lore.kernel.org/linux-mm/1b27a3fa-359a-43d0-bdeb-c31341749367@xxxxxxxxxx/
> 
> Link: https://lore.kernel.org/linux-mm/20260202150957.GD1282955@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> Link: https://lore.kernel.org/linux-mm/dfdfeac9-5cd5-46fc-a5c1-9ccf9bd3502a@xxxxxxxxx/
> Link: https://lore.kernel.org/linux-mm/bc489455-bb18-44dc-8518-ae75abda6bec@xxxxxxxxxx/
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx>
> Suggested-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
> ---
> v1 -> v2:
> - Wrap synchronize_rcu() in tlb_remove_table_sync_rcu() with proper
>   kerneldoc (per David)
> - Add might_sleep() to make sleeping constraint explicit (per Dave)
> - Clarify this is for synchronization, not memory freeing (per Dave)
> - https://lore.kernel.org/linux-mm/20260223033604.10198-1-lance.yang@xxxxxxxxx/
> 
>  include/asm-generic/tlb.h |  4 ++++
>  mm/mmu_gather.c           | 22 +++++++++++++++++++++-
>  2 files changed, 25 insertions(+), 1 deletion(-)
> 
> 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

Nit: no need for the "()"

Thanks!

Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

-- 
Cheers,

David