Re: [PATCH v2 06/11] mm/sparse-vmemmap: switch device DAX to shared tail vmemmap pages

[Qi Zheng <qi.zheng@linux.dev>]

On 9/8/26 11:03 AM, 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.
> 
> The optimization is enabled only for DEV-DAX through pgmap->vmemmap_shift,
> which is assigned when a DEV-DAX device is initialized. Unlike FS-DAX,
> DEV-DAX does not modify tail struct pages, so sharing them is safe.

Got it — adding this description makes things significantly clearer.

> 
> Since the shared tail page can now back ZONE_DEVICE vmemmap mappings,
> initialize its entries with PG_reserved for device zones. Also skip
> poisoning vmemmap-optimizable sections while their struct pages may be
> shared.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
> v2:
> - Explain why sharing tail vmemmap pages is safe for DEV-DAX
>    (suggested by Qi Zheng)
> ---
>   include/linux/mmzone.h | 10 +++++++++
>   mm/memory_hotplug.c    |  5 +++--
>   mm/sparse-vmemmap.c    | 47 ++++++++++++++----------------------------
>   3 files changed, 28 insertions(+), 34 deletions(-)
> 

Acked-by: Qi Zheng <qi.zheng@linux.dev>

Thanks,
Qi