Re: [PATCH] arm64: mm: Fix pfn_is_map_memory() misreporting mapped memory

[Vladimir Murzin <vladimir.murzin@arm.com>]

On 9/9/26 14:31, Will Deacon wrote:
> On Tue, Sep 08, 2026 at 05:20:33PM +0100, Vladimir Murzin wrote:
>> Commit 7ace06a01efa ("arm64: mm: fix accidental linear mapping of
>> no-map reserved memory") removed sub-page no-map regions from the
>> linear mapping. However, pfn_is_map_memory() can still report that
>> such a region is mapped.
>>
>> For instance, with 64K pages, say we have
>>
>> normal:        ...–0xa2007fff
>> no-map: 0xa2008000–0xa200ffff
>> normal: 0xa2010000–...
>>
>> The range 0xa2000000–0xa200ffff is not linearly mapped, but
>> pfn_is_map_memory(__phys_to_pfn(0xa2008000)) aligns the address to
>> page granularity and passes 0xa2000000 to memblock_is_map_memory().
>> That address belongs to the preceding normal memblock region, so the
>> no-map region is ignored and the function returns true.
>>
>> Fix that by checking if entire page is subset of memory block.
>>
>> Fixes: 7ace06a01efa ("arm64: mm: fix accidental linear mapping of no-map reserved memory")
>> Assisted-by: LLM
>> Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
>> ---
>>  arch/arm64/mm/init.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
>> index fbf215ecc7d0..5da80e1e1727 100644
>> --- a/arch/arm64/mm/init.c
>> +++ b/arch/arm64/mm/init.c
>> @@ -172,7 +172,8 @@ int pfn_is_map_memory(unsigned long pfn)
>>  	if (PHYS_PFN(addr) != pfn)
>>  		return 0;
>>  
>> -	return memblock_is_map_memory(addr);
>> +	return memblock_is_region_memory(addr, PAGE_SIZE) &&
>> +		memblock_is_map_memory(addr);
>>  }
> Yuck, this is really grotty :(
> 
> Can we at least predicate the extra memblock_is_region_memory() check on
> having a page-size > 4k?
> 

The same can happen with 4K page size and subpage no-map and I have not seen
anything prohibiting subpage no-map ...

Cheers
Vladimir

> Will
>