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

[David Hildenbrand (Arm)]

On 3/5/26 13:37, David Hildenbrand (Arm) wrote:
> 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>
> 

In fact,

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

:)

-- 
Cheers,

David