Re: [PATCH RFC 1/3] mm/tlb: allow architectures to skip redundant TLB sync IPIs

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

On 12/15/25 06:48, Lance Yang wrote:
> 
> 
> On 2025/12/13 16:00, Lance Yang wrote:
>> From: Lance Yang <lance.yang@linux.dev>
>>
>> When unsharing hugetlb PMD page tables, we currently send two IPIs:
>> one for TLB invalidation, and another to synchronize with concurrent
>> GUP-fast walkers.
>>
>> However, if the TLB flush already reaches all CPUs, the second IPI is
>> redundant. GUP-fast runs with IRQs disabled, so when the TLB flush IPI
>> completes, any concurrent GUP-fast must have finished.
>>
>> Add tlb_table_flush_implies_ipi_broadcast() to let architectures indicate
>> their TLB flush provides full synchronization, enabling the redundant IPI
>> to be skipped.
>>
>> The default implementation returns false to maintain current behavior.
>>
>> Suggested-by: David Hildenbrand (Red Hat) <david@kernel.org>
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>> ---
>>    include/asm-generic/tlb.h | 22 +++++++++++++++++++++-
>>    1 file changed, 21 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
>> index 324a21f53b64..3f0add95604f 100644
>> --- a/include/asm-generic/tlb.h
>> +++ b/include/asm-generic/tlb.h
>> @@ -248,6 +248,21 @@ static inline void tlb_remove_table(struct mmu_gather *tlb, void *table)
>>    #define tlb_needs_table_invalidate() (true)
>>    #endif
>>    
>> +/*
>> + * Architectures can override if their TLB flush already broadcasts IPIs to all
>> + * CPUs when freeing or unsharing page tables.
>> + *
>> + * Return true only when the flush guarantees:
>> + * - IPIs reach all CPUs with potentially stale paging-structure cache entries
>> + * - Synchronization with IRQ-disabled code like GUP-fast
>> + */
>> +#ifndef tlb_table_flush_implies_ipi_broadcast
>> +static inline bool tlb_table_flush_implies_ipi_broadcast(void)
>> +{
>> +	return false;
>> +}
>> +#endif
> 
> As the kernel test robot reported[1][2], the compiler is unhappy with
> patch #3:
> 
> ```
>      mm/khugepaged.c: In function 'collapse_huge_page':
>>>>> mm/khugepaged.c:1185:14: error: implicit declaration of function 'tlb_table_flush_implies_ipi_broadcast' [-Werror=implicit-function-declaration]
>       1185 |         if (!tlb_table_flush_implies_ipi_broadcast())
>            |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>      cc1: some warnings being treated as errors
> ```
> 
> I'll move tlb_table_flush_implies_ipi_broadcast() outside of
> CONFIG_MMU_GATHER_RCU_TABLE_FREE in next version, making the complier
> happy on architectures that don't enable that config ;)

Yeah, that's probably cleanest.

-- 
Cheers

David