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

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Message-ID: <f7552c2e-e12c-47c8-9b60-ec645cddf804@kernel.org>
Date: Thu, 18 Dec 2025 14:08:50 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org
Cc: will@...nel.org, aneesh.kumar@...nel.org, npiggin@...il.com,
 peterz@...radead.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, arnd@...db.de,
 lorenzo.stoakes@...cle.com, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
 Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com,
 dev.jain@....com, baohua@...nel.org, ioworker0@...il.com,
 shy828301@...il.com, riel@...riel.com, jannh@...gle.com,
 linux-arch@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 1/3] mm/tlb: allow architectures to skip redundant TLB
 sync IPIs

On 12/15/25 06:48, Lance Yang wrote:
> 
> 
> On 2025/12/13 16:00, Lance Yang wrote:
>> From: Lance Yang <lance.yang@...ux.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@...nel.org>
>> Signed-off-by: Lance Yang <lance.yang@...ux.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