Re: [PATCH V4 2/2] arm64/mm: Reject memory removal that splits a kernel leaf mapping

[David Hildenbrand (Arm)]

On 3/5/26 06:31, Anshuman Khandual wrote:
> Linear and vmemmap mappings that get torn down during a memory hot remove
> operation might contain leaf level entries on any page table level. If the
> requested memory range's linear or vmemmap mappings falls within such leaf
> entries, new mappings need to be created for the remaining memory mapped on
> the leaf entry earlier, following standard break before make aka BBM rules.
> But kernel cannot tolerate BBM and hence remapping to fine grained leaves
> would not be possible on systems without BBML2_NOABORT.
> 
> Currently memory hot remove operation does not perform such restructuring,
> and so removing memory ranges that could split a kernel leaf level mapping
> need to be rejected.
> 
> While memory_hotplug.c does appear to permit hot removing arbitrary ranges
> of memory, the higher layers that drive memory_hotplug (e.g. ACPI, virtio,
> ...) all appear to treat memory as fixed size devices. So it is impossible
> to hot unplug a different amount than was previously hot plugged, and hence
> we should never see a rejection in practice, but adding the check makes us
> robust against a future change.
> 
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Link: https://lore.kernel.org/all/aWZYXhrT6D2M-7-N@willie-the-truck/
> Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> Suggested-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ---

[...]

> +
> +	/*
> +	 * PFN range's linear map edges are leaf entry aligned
> +	 */

Nit: single-line comments are usually written as

/* ... */

> +	start = __phys_to_virt(phys_start);
> +	end =  __phys_to_virt(phys_end);
> +	if (addr_splits_kernel_leaf(start) || addr_splits_kernel_leaf(end)) {
> +		pr_warn("[%lx %lx] splits a leaf entry in linear map\n",
> +			phys_start, phys_end);
> +		return false;
> +	}
> +
> +	/*
> +	 * PFN range's vmemmap edges are leaf entry aligned
> +	 */

Dito.

LGTM, thanks!

Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

-- 
Cheers,

David