Re: [PATCH v5 07/12] mm/sparse-vmemmap: switch device DAX to shared tail vmemmap pages

On 9/27/26 04:54, 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 supplies the compound page order recorded in section metadata before
vmemmap population. Unlike FS-DAX, DEV-DAX does not modify tail struct
pages, so sharing them is safe.

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 <redacted>
Acked-by: Qi Zheng <qi.zheng@linux.dev>
---
v3:
- Move device_zone() after the definition of NODE_DATA() to fix
  non-NUMA builds.
- Update the commit message to describe the compound page order stored
  in section metadata
- Collect Acked-by from Qi Zheng

v2:
- Explain why sharing tail vmemmap pages is safe for DEV-DAX
  (suggested by Qi Zheng)
---

[...]

const and all the way to the top.


I did wonder about the poisoning change ... because the memmap usually gets
initialized once the memory section gets moved to a zone.

SO now I'm a bit confused about the ordering of events :)

-- 
Cheers,

David