Re: [PATCH 06/11] mm/sparse-vmemmap: switch device DAX to shared tail vmemmap pages

[Qi Zheng]

On 8/31/26 3:53 PM, Muchun Song wrote:
> HugeTLB vmemmap optimization now uses per-zone shared tail vmemmap pages.
> Device DAX has not been switched to that mechanism yet.
>
> Switch device DAX to vmemmap_shared_tail_page() as well. This aligns DAX
> with HugeTLB by using the common per-zone shared tail vmemmap page.
>
> Since the shared tail page can now back ZONE_DEVICE vmemmap mappings,

Could you explain this in more detail?

As sashiko pointed out [1], it looks like this assumption is
problematic: dax_folio_reset_order() may end up modifying all shared
tail pages?

[1]. 
https://sashiko.dev/#/patchset/20260831075342.57563-1-songmuchun%40bytedance.com

> initialize its entries with PG_reserved for device zones. Also skip
> poisoning vmemmap-optimizable sections while their struct pages may be
> shared.