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

[Mike Rapoport <rppt@kernel.org>]

On Thu, Sep 17, 2026 at 02:34:57PM +0200, Ard Biesheuvel wrote:
> On Thu, 17 Sep 2026, at 14:26, Vladimir Murzin wrote:
> > 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()?
> >
> 
> Yes.
> 
> > This is indeed much nicer way to fix the problem! Now, when no-map is
> > rounded outwards, do we still need 7ace06a01efa?
> >
> 
> Probably not, but there are some corner cases to consider before we
> go down this route:
> - what happens when marking a range no-map where the outward rounding
>   would exceed the limits of the existing memblock memory range?

AFAIR it's fine, memblock_isolate will clamp the update.
Didn't check the code though.

> - what happens when removing a non-page aligned region from memblock
>   that intersects with a (page aligned) no-map region?

It will make the nomap region non-page aligned.

-- 
Sincerely yours,
Mike.