linux-kernel - Re: [PATCH RESEND v3 1/2] mm/tlb: skip redundant IPI when TLB flush already synchronized

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Message-ID: <da1e8a00-99fe-46d9-b425-c307ea933036@kernel.org>
Date: Tue, 6 Jan 2026 16:19:10 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org
Cc: dave.hansen@...el.com, dave.hansen@...ux.intel.com, will@...nel.org,
 aneesh.kumar@...nel.org, npiggin@...il.com, peterz@...radead.org,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 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,
 shy828301@...il.com, riel@...riel.com, jannh@...gle.com,
 linux-arch@...r.kernel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, ioworker0@...il.com
Subject: Re: [PATCH RESEND v3 1/2] mm/tlb: skip redundant IPI when TLB flush
 already synchronized

>   
>   static void tlb_table_flush(struct mmu_gather *tlb)
> @@ -367,7 +378,7 @@ void tlb_remove_table(struct mmu_gather *tlb, void *table)
>   		*batch = (struct mmu_table_batch *)__get_free_page(GFP_NOWAIT);
>   		if (*batch == NULL) {
>   			tlb_table_invalidate(tlb);
> -			tlb_remove_table_one(table);
> +			tlb_remove_table_one(table, tlb);
>   			return;
>   		}
>   		(*batch)->nr = 0;
> @@ -427,6 +438,7 @@ static void __tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm,
>   	tlb->vma_pfn = 0;
>   
>   	tlb->fully_unshared_tables = 0;
> +	tlb->tlb_flush_sent_ipi = 0;
>   	__tlb_reset_range(tlb);
>   	inc_tlb_flush_pending(tlb->mm);
>   }

But when would we have to reset tlb->tlb_flush_sent_ipi = 0 later? 
That's where it gets tricky. Just imagine the MMU gather gets reused later.

Also,

	+	if (info->freed_tables && info->tlb)
	+		info->tlb->tlb_flush_sent_ipi = true;

in native_flush_tlb_multi() misses the fact that we have different 
flushing types for removed/unshared tables vs. other flush.

So this approach more here certainly gets more complicated and error prone.

tlb_table_flush_implies_ipi_broadcast() was clearer in that regard: if 
you flushed the TLB after removing /unsharing tables, the IPI for 
handling page tables can be skipped. It's on the code flow to assure that.

What could work is tracking "tlb_table_flush_sent_ipi" really when we 
are flushing the TLB for removed/unshared tables, and maybe resetting it 
... I don't know when from the top of my head.

v2 was simpler IMHO.

-- 
Cheers

David