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

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Message-ID: <725b85bf-ff5e-45d6-991e-d92598779f98@kernel.org>
Date: Tue, 30 Dec 2025 21:31:49 +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 v2 1/3] mm/tlb: allow architectures to skip redundant TLB
 sync IPIs

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.

-- 
Cheers

David