mm/sparse-vmemmap: set compound page order for device DAX

Device DAX can use vmemmap optimization only when a full section is
populated with a compound-page geometry.  Record that geometry as the
compound page order in section metadata before populating the section, so
later vmemmap accounting and population decisions can use the section
state directly.

Clear the compound page order when the section becomes empty again.  Also
reject partial additions to a section that already has optimized vmemmap
mappings.  compound_nr_pages() determines how many struct pages to
initialize with a section as the smallest granularity.  A section
therefore cannot safely mix optimized and ordinary vmemmap layouts.

Partial additions continue to use ordinary vmemmap population, so they do
not save vmemmap memory.  Such additions are uncommon, and the lost saving
is negligible.

Link: https://lore.kernel.org/20260930140627.57431-7-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>
Acked-by: David Hildenbrand (Arm) <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