linux-kernel - Re: [PATCH RFC 3/3] mm/khugepaged: skip redundant IPI in collapse_huge

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Message-ID: <948d425a-2d6e-4439-a280-0ca9e7521b13@kernel.org>
Date: Thu, 18 Dec 2025 14:13:09 +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 3/3] mm/khugepaged: skip redundant IPI in
 collapse_huge_page()

On 12/13/25 09:00, Lance Yang wrote:
> From: Lance Yang <lance.yang@...ux.dev>
> 
> Similar to the hugetlb PMD unsharing optimization, skip the second IPI
> in collapse_huge_page() when the TLB flush already provides necessary
> synchronization.
> 
> Before commit a37259732a7d ("x86/mm: Make MMU_GATHER_RCU_TABLE_FREE
> unconditional"), bare metal x86 didn't enable MMU_GATHER_RCU_TABLE_FREE.
> In that configuration, tlb_remove_table_sync_one() was a NOP. GUP-fast
> synchronization relied on IRQ disabling, which blocks TLB flush IPIs.
> 
> When Rik made MMU_GATHER_RCU_TABLE_FREE unconditional to support AMD's
> INVLPGB, all x86 systems started sending the second IPI. However, on
> native x86 this is redundant:
> 
>    - pmdp_collapse_flush() calls flush_tlb_range(), sending IPIs to all
>      CPUs to invalidate TLB entries
> 
>    - GUP-fast runs with IRQs disabled, so when the flush IPI completes,
>      any concurrent GUP-fast must have finished
> 
>    - tlb_remove_table_sync_one() provides no additional synchronization
> 
> On x86, skip the second IPI when running native (without paravirt) and
> without INVLPGB. For paravirt with non-native flush_tlb_multi and for
> INVLPGB, conservatively keep both IPIs.
> 
> Use tlb_table_flush_implies_ipi_broadcast(), consistent with the hugetlb
> optimization.
> 
> Suggested-by: David Hildenbrand (Red Hat) <david@...nel.org>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
> ---
>   mm/khugepaged.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 97d1b2824386..06ea793a8190 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1178,7 +1178,12 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>   	_pmd = pmdp_collapse_flush(vma, address, pmd);
>   	spin_unlock(pmd_ptl);
>   	mmu_notifier_invalidate_range_end(&range);
> -	tlb_remove_table_sync_one();
> +	/*
> +	 * Skip the second IPI if the TLB flush above already synchronized
> +	 * with concurrent GUP-fast via broadcast IPIs.
> +	 */
> +	if (!tlb_table_flush_implies_ipi_broadcast())
> +		tlb_remove_table_sync_one();

We end up calling

	flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE);

	-> flush_tlb_mm_range(freed_tables = true)

	-> flush_tlb_multi(mm_cpumask(mm), info);

So freed_tables=true and we should be doing the right thing.

BTW, I was wondering whether we should embed that 
tlb_table_flush_implies_ipi_broadcast() check in 
tlb_remove_table_sync_one() instead.

It then relies on the caller to do the right thing (flush with 
freed_tables=true or unshared_tables = true).

Thoughts?

-- 
Cheers

David