Re: [PATCH V4 1/2] arm64/mm: Enable batched TLB flush in unmap_hotplug_range()

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/5/26 06:31, Anshuman Khandual wrote:
> During a memory hot remove operation, both linear and vmemmap mappings for
> the memory range being removed, get unmapped via unmap_hotplug_range() but
> mapped pages get freed only for vmemmap mapping. This is just a sequential
> operation where each table entry gets cleared, followed by a leaf specific
> TLB flush, and then followed by memory free operation when applicable.
> 
> This approach was simple and uniform both for vmemmap and linear mappings.
> But linear mapping might contain CONT marked block memory where it becomes
> necessary to first clear out all entire in the range before a TLB flush.
> This is as per the architecture requirement. Hence batch all TLB flushes
> during the table tear down walk and finally do it in unmap_hotplug_range().
> 
> Prior to this fix, it was hypothetically possible for a speculative access
> to a higher address in the contiguous block to fill the TLB with shattered
> entries for the entire contiguous range after a lower address had already
> been cleared and invalidated. Due to the table entries being shattered, the
> subsequent TLB invalidation for the higher address would not then clear the
> TLB entries for the lower address, meaning stale TLB entries could persist.
> 
> Besides it also helps in improving the performance via TLBI range operation
> along with reduced synchronization instructions. The time spent executing
> unmap_hotplug_range() improved 97% measured over a 2GB memory hot removal
> in KVM guest.
> 
> This scheme is not applicable during vmemmap mapping tear down where memory
> needs to be freed and hence a TLB flush is required after clearing out page
> table entry.
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Closes: https://lore.kernel.org/all/aWZYXhrT6D2M-7-N@willie-the-truck/
> Fixes: bbd6ec605c0f ("arm64/mm: Enable memory hot remove")
> Cc: stable@vger.kernel.org
> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
>  arch/arm64/mm/mmu.c | 46 +++++++++++++++++++++++++++++----------------
>  1 file changed, 30 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index a6a00accf4f9..f7ccda22d39e 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1458,10 +1458,18 @@ static void unmap_hotplug_pte_range(pmd_t *pmdp, unsigned long addr,
>  
>  		WARN_ON(!pte_present(pte));
>  		__pte_clear(&init_mm, addr, ptep);
> -		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> -		if (free_mapped)
> +		if (free_mapped) {
> +			/*
> +			 * CONT blocks are not supported in the vmemmap
> +			 */

Same nit as on patch #2,

/* CONT blocks are not supported in the vmemmap */


Same for the other comments.

Thanks!

Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David