mm/sparse-vmemmap: prepare DAX vmemmap population for compound page orders

Device DAX still uses vmemmap_populate_compound_pages() to populate its
compound-page vmemmap mappings.  That helper allocates the head and first
tail vmemmap pages explicitly, then reuses the first tail page for the
remaining tail page mappings.

Device DAX is being moved to the section-based vmemmap optimization
infrastructure, but it cannot switch to the generic section-based
population path yet.  Once a later patch records the DAX compound page
order in section metadata, DAX head and first-tail PFNs can look
optimizable to the generic helpers as well.

Rename the existing VMEMMAP_POPULATE_PAGEREF flag to VMEMMAP_POPULATE_DAX
and pass it through all paths in vmemmap_populate_compound_pages() during
this transition.  This keeps DAX head/first-tail allocations on the normal
vmemmap allocation path, while preserving the existing page reference for
reused DAX tail mappings.

Link: https://lore.kernel.org/20260930140627.57431-6-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