Re: [PATCH 6.1 337/522] arm64/mm: Enable batched TLB flush in unmap_hotplug_range()

[Ben Hutchings]

On Tue, 2026-06-16 at 20:28 +0530, Greg Kroah-Hartman wrote:
> 6.1-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> 
> [ Upstream commit 48478b9f791376b4b89018d7afdfd06865498f65 ]
[...]
> @@ -949,15 +953,14 @@ static void unmap_hotplug_pmd_range(pud_
>  		WARN_ON(!pmd_present(pmd));
>  		if (pmd_sect(pmd)) {
>  			pmd_clear(pmdp);
> -
> -			/*
> -			 * One TLBI should be sufficient here as the PMD_SIZE
> -			 * range is mapped with a single block entry.
> -			 */
> -			flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> -			if (free_mapped)
> +			if (free_mapped) {
> +				/* CONT blocks are not supported in the vmemmap */
> +				WARN_ON(pmd_cont(pmd));
> +				flush_tlb_kernel_range(addr, addr + PMD_SIZE);

It wasn't clear to me from the commit message why this now adds PMD_SIZE
rather than PAGE_SIZE.  It seems like this change is fine for Linux
6.13+ with a CPU that supports TLB range flushing, but otherwise results
in unnecessarily executing multiple TLB invalidations at intervals of
the base page size.

>  				free_hotplug_page_range(pmd_page(pmd),
>  							PMD_SIZE, altmap);
> +			}
> +			/* unmap_hotplug_range() flushes TLB for !free_mapped */
>  			continue;
>  		}
>  		WARN_ON(!pmd_table(pmd));
> @@ -982,15 +985,12 @@ static void unmap_hotplug_pud_range(p4d_
>  		WARN_ON(!pud_present(pud));
>  		if (pud_sect(pud)) {
>  			pud_clear(pudp);
> -
> -			/*
> -			 * One TLBI should be sufficient here as the PUD_SIZE
> -			 * range is mapped with a single block entry.
> -			 */
> -			flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> -			if (free_mapped)
> +			if (free_mapped) {
> +				flush_tlb_kernel_range(addr, addr + PUD_SIZE);
[...]

Similarly here, but this is effectively flush_tlb_all() instead.

Ben.

-- 
Ben Hutchings
No political challenge can be met by shopping. - George Monbiot

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