> On Sep 28, 2026, at 03:54, Andrew Morton <[email protected]> wrote:
>
> On Sun, 27 Sep 2026 18:51:15 +0800 Muchun Song <[email protected]> wrote:
>
>>
>>
>>> On Sep 27, 2026, at 13:51, Andrew Morton <[email protected]> wrote:
>>>
>>> On Sun, 27 Sep 2026 10:54:29 +0800 Muchun Song <[email protected]>
>>> wrote:
>>>
>>>> After the HugeTLB conversion, optimized vmemmap state is described by
>>>> the memory section and the sparse-vmemmap population path can allocate or
>>>> reuse shared tail vmemmap pages based on that metadata. Device DAX still
>>>> uses the older DAX-specific population model, including a separate tail
>>>> vmemmap page reservation and architecture-specific logic to locate or
>>>> populate reusable tail pages.
>>>>
>>>> This series makes device DAX use the same section-based model. Device DAX
>>>> records the compound page order from pgmap->vmemmap_shift in section
>>>> metadata before vmemmap population, uses the common per-zone shared tail
>>>> vmemmap page, and drops the extra reserved tail page. The powerpc radix
>>>> path is updated to use the same shared tail-page helper, so the generic
>>>> and powerpc DAX paths follow the same reservation model.
>>>
>>> Thanks, I've updated mm-unstable to this version.
>>
>> Thanks.
>>
>>>
>>> Sashiko asked a thing:
>>> https://sashiko.dev/#/patchset/[email protected]
>>
>> Sashiko said page->refcount can overflow by incrementing it over 2.14 billion
>> times when mapping more than **524 TB** of DEV-DAX memory on a single NUMA
>> node, where the pages share the same node, order, and zone.
>>
>> I am not aware of any practical hardware configuration approaching this
>> topology today.
>>
>> Handling that theoretical limit would add non-trivial lifetime or
>> architecture-specific teardown complexity. Without a concrete hardware
>> requirement, I prefer not to over-engineer the current series. We can revisit
>> it when such a system or use case becomes realistic.
>
> OK. Presumably it would be cheap to add a check for this craziness and
> return ENOSOMETHING?
Sounds right — I'll send a follow-up fixup patch shortly.
Muchun,
Thanks.
