mm/sparse-vmemmap: switch device DAX to shared tail vmemmap pages

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.

Each PTE mapping the shared device DAX tail page takes a page reference. 
A sufficiently large range could therefore cycle the reference count back
to zero if population were allowed to continue after it became
non-positive.  Use try_get_page() so further mappings fail at that point. 
The section population error path tears down mappings created for the
failed section, while the warning makes this currently impractical limit
visible.

[muchun.song@linux.dev: set PG_reserved before preparing the compound tail]
  Link: https://lore.kernel.org/20260930150748.1134516-1-songmuchun@bytedance.com
Link: https://lore.kernel.org/20260930140627.57431-8-songmuchun@bytedance.com
Signed-off-by: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Qi Zheng <qi.zheng@linux.dev>
Cc: David Hildenbrand <david@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Christophe Leroy <chleroy@kernel.org>
Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Lance Yang <lance.yang@linux.dev>

0 files changed, 0 insertions, 0 deletions