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

[Will Deacon <will@kernel.org>]

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?

Will