linux-kernel - Re: [PATCH v2 1/3] mm/tlb: allow architectures to skip redundant TLB sync IPIs

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Message-ID: <af13561e-c512-4ab3-af5f-3b2057ac6667@linux.dev>
Date: Wed, 31 Dec 2025 10:29:29 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>,
 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 v2 1/3] mm/tlb: allow architectures to skip redundant TLB
 sync IPIs



On 2025/12/31 04:31, David Hildenbrand (Red Hat) wrote:
> On 12/29/25 15:52, 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.
>>
>> Suggested-by: David Hildenbrand (Red Hat) <david@...nel.org>
>> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
>> ---
>>   include/asm-generic/tlb.h | 14 ++++++++++++++
>>   1 file changed, 14 insertions(+)
>>
>> diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
>> index 4d679d2a206b..e8d99b5e831f 100644
>> --- a/include/asm-generic/tlb.h
>> +++ b/include/asm-generic/tlb.h
>> @@ -261,6 +261,20 @@ static inline void 
>> tlb_remove_table_sync_one(void) { }
>>   #endif /* CONFIG_MMU_GATHER_RCU_TABLE_FREE */
>> +/*
>> + * 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
>>   #ifndef CONFIG_MMU_GATHER_NO_GATHER
>>   /*
> 
> 
> This should likely get squashed into patch #3. Patch #1 itself does not 
> add a lot of value to be had separately.
> 
> So best to squash both and have them as #1, to then implement it in #2 
> for x86.

Sounds good, will do! Squashing #1 and #3 together, keeping the x86
implementation as #2 ;)

Cheers,
Lance