mm/collapse: add collapse.h for the collapse interface

khugepaged.c holds both the users of collapse and the machinery that
performs it.  The daemon's scan loop, the sysfs tunables, MADV_COLLAPSE's
entry point and the collapse itself all sit in one file and reach into
each other freely.  Nothing marks where a user ends and the engine
begins.

Start drawing that line.

Add mm/collapse.h for what the two sides have to agree on:

  - enum scan_result - what the engine hands back;
  - struct collapse_control - the state a request carries.

And two constants move with them:

  - KHUGEPAGED_MAX_PTES_LIMIT -> COLLAPSE_MAX_PTES_LIMIT;
  - KHUGEPAGED_MIN_MTHP_ORDER -> COLLAPSE_MIN_MTHP_ORDER.

Neither is a fact about the daemon, so both lose the KHUGEPAGED_ prefix.

No functional change.

Link: https://lore.kernel.org/20260928100630.21870-5-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Assisted-by: LLM
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Jann Horn <jannh@google.com>

0 files changed, 0 insertions, 0 deletions