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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 2/23/26 04:36, 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.
> synchronize_rcu() waits for all such sections to complete, providing the
> same guarantee as IPI but without disrupting all CPUs.
> 
> Since batch allocation already failed, we are in a way slow path, so
> replacing the IPI with synchronize_rcu() is fine.
> 
> We are in process context (unmap_region, exit_mmap) with only mmap_lock
> held, a sleeping lock. synchronize_rcu() will catch any invalid context
> via might_sleep().
> 
> [1] https://lore.kernel.org/linux-mm/1b27a3fa-359a-43d0-bdeb-c31341749367@kernel.org/
> 
> Link: https://lore.kernel.org/linux-mm/20260202150957.GD1282955@noisy.programming.kicks-ass.net/
> Link: https://lore.kernel.org/linux-mm/dfdfeac9-5cd5-46fc-a5c1-9ccf9bd3502a@intel.com/
> Link: https://lore.kernel.org/linux-mm/bc489455-bb18-44dc-8518-ae75abda6bec@kernel.org/
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Suggested-by: Dave Hansen <dave.hansen@intel.com>
> Suggested-by: David Hildenbrand (Arm) <david@kernel.org>

I think it was primarily Peter and Dave suggesting that :)

> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> ---
>   mm/mmu_gather.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
> index fe5b6a031717..df670c219260 100644
> --- a/mm/mmu_gather.c
> +++ b/mm/mmu_gather.c
> @@ -339,7 +339,8 @@ static inline void __tlb_remove_table_one(void *table)
>   #else
>   static inline void __tlb_remove_table_one(void *table)
>   {
> -	tlb_remove_table_sync_one();
> +	if (IS_ENABLED(CONFIG_MMU_GATHER_RCU_TABLE_FREE))
> +		synchronize_rcu();

That should work.

Reading all the comments for tlb_remove_table_smp_sync(), I wonder 
whether we should wrap that in a tlb_remove_table_sync_rcu() function, 
with a proper kerneldoc for the CONFIG_MMU_GATHER_RCU_TABLE_FREE variant 
where we discuss how this relates to tlb_remove_table_sync_one (and 
tlb_remove_table_smp_sync() .

-- 
Cheers,

David