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.

Add a DAX-specific population flag for this transition.  It 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/20260911050228.58884-5-songmuchun@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Qi Zheng <qi.zheng@linux.dev>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Randy Dunlap <rdunlap@infradead.org>

0 files changed, 0 insertions, 0 deletions