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

[Dev Jain <dev.jain@arm.com>]

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.


>