Re: [PATCH v3 1/6] arm64: make huge_ptep_get handled unaligned addresses

["David Hildenbrand (Arm)" <david@kernel.org>]

On 7/5/26 10:08, Dev Jain wrote:
> 
> 
> On 05/07/26 1:05 pm, Andrew Morton wrote:
>> On Fri,  3 Jul 2026 11:41:54 +0000 Dev Jain <dev.jain@arm.com> wrote:
>>
>>> huge_ptep_get() can be handed a virtual address pointing to the middle of
>>> a contpmd/contpte mapped hugetlb folio (examples of callers are
>>> pagemap_hugetlb_range, page_mapped_in_vma).
>>>
>>> The arm64 helper rewalks the pgtables in find_num_contig to answer whether
>>> the huge pte we have maps a contpmd or a contpte hugetlb folio, and
>>> returns CONT_PMDS or CONT_PTES, so that it can collect a/d bits over the
>>> contiguous ptes. We can falsely return CONT_PTES instead of CONT_PMDS
>>> if the addr is not aligned.
>>>
>>> Fix this by aligning the pmdp pointer down to a contpmd base before
>>> checking equality with the passed huge pte pointer, to correctly answer
>>> whether the huge pte is the base of a contpmd block.
>>>
>>> Fixes: 29cb80519689 ("arm64: hugetlb: Cleanup huge_pte size discovery mechanisms")
>>> Cc: stable@vger.kernel.org
>>
>> Please describe the userspace-visible effects of bugs when fixing them.
>> Particularly when cc:stable is proposed.  Thanks.
> 
> Forgot for this one. It should be, on systems where CONT_PTES != CONT_PMDS
> (meaning page size is 16K) we could collect excess a/d bit state, meaning
> extra work for the kernel.

Even worse, right? We could walk 128 entries, when we really should just walk 16
(IIRC) entries, possibly reading garbage or even worse, into a memory hole at
the end of memory?

-- 
Cheers,

David