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

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

On 9/16/26 11:13, Ard Biesheuvel wrote:
> (cc Mike)
> 
> On Tue, 8 Sep 2026, at 18:20, 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);
>>  }
>>  EXPORT_SYMBOL(pfn_is_map_memory);
>>
> memblock_is_region_memory() only tells you whether the range in
> question is covered by a single entry in memblock.memory.regions[].
> 
> memblock has other flags, which may or may not be set on sub-page
> regions too. So I don't think this is the right solution in the
> general case, even if it fixes your example.
> 

Ack.

> Given that the no-map attribute fundamentally only applies to page
> granular regions, it would make sense to round those outwards
> whenever they are created.
> 
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -1119,6 +1119,10 @@
>  int __init_memblock memblock_mark_nomap(phys_addr_t base, phys_addr_t size)
>  {
> +       phys_addr_t end = PAGE_ALIGN(base + size);
> +
> +       base &= PAGE_MASK;
> +       size = end - base;
>         return memblock_setclr_flag(&memblock.memory, base, size, 1, MEMBLOCK_NOMAP);
>  }
>  
> 

Should the same be applied to memblock_clear_nomap()?

This is indeed much nicer way to fix the problem! Now, when no-map is
rounded outwards, do we still need 7ace06a01efa?

Thanks
Vladimir

> Note that I recently removed the frivolous use of MEMBLOCK_NOMAP in
> __map_mem() so the remaining calls should mostly be true reservations
> originating from the boot environment.
> 
> 
> 
> 
>