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

["Ard Biesheuvel" <ardb@kernel.org>]

(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.

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);
 }
 

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.