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

[Mike Rapoport]

On Thu, Sep 17, 2026 at 05:17:45PM +0200, Ard Biesheuvel wrote:
> On Thu, 17 Sep 2026, at 16:29, Mike Rapoport wrote:
> > 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 at 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?
> >> - what happens when removing a non-page aligned region from memblock
> >>   that intersects with a (page aligned) no-map region?
> >
> > Another thing is that maybe we should force memblock.memory only to page
> > aligned ranges. x86 uses memblock_trim_memory() for that since, like,
> > forever.
> >
> 
> Interesting. It would make sense to do the same on arm64.
> 
> But if a NOMAP region ends in the middle of a page, that code will trim on
> both sides, and throw away the shared page entirely.

Honestly, I think the whole sub-page NOMAP is firmware's problem. While
rounding up in _mark_nomap() makes sense because we cannot "not map" a
sub-page range, I would add a big fat WARN_ON("FIX YOUR FIRMWARE") if the
rounding actually happens.

As for the trimming on arm64, I believe that's relevant, because again, we
cannot really work with sub-page ranges in memblock.memory and they are
anyway discarded by for_each_mem_pfn_range() that's used all over mm
initialization.

> So all flag manipulation occurring beforehand should be rounded outward
> (not just NOMAP, although I'm not sure if there are others that may get
> set on non-page granular regions)

Virtually anything, but I don't think any existing caller does that.

-- 
Sincerely yours,
Mike.