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

[Lance Yang <lance.yang@linux.dev>]

On 2026/2/24 19:04, David Hildenbrand (Arm) wrote:
> On 2/24/26 04:07, 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.
>>
>> [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>
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>> ---
>> 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@linux.dev/
>>
>>   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 "()"

Oops, @Andrew could you drop the "()" here when applying?

> 
> Thanks!
> 
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>

Thanks for taking time to review!