mm/cma: introduce interface for early reservations

mm/cma: introduce interface for early reservations

It can be desirable to reserve memory in a CMA area before it is
activated, early in boot.  Such reservations would effectively be memblock
allocations, but they can be returned to the CMA area later.  This
functionality can be used to allow hugetlb bootmem allocations from a
hugetlb CMA area.

A new interface, cma_reserve_early is introduced.  This allows for
pageblock-aligned reservations.  These reservations are skipped during the
initial handoff of pages in a CMA area to the buddy allocator.  The caller
is responsible for making sure that the page structures are set up, and
that the migrate type is set correctly, as with other memblock allocations
that stick around.  If the CMA area fails to activate (because it
intersects with multiple zones), the reserved memory is not given to the
buddy allocator, the caller needs to take care of that.

Link: https://lkml.kernel.org/r/20250228182928.2645936-25-fvdl@google.com
Signed-off-by: Frank van der Linden <fvdl@google.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Joao Martins <joao.m.martins@oracle.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Roman Gushchin (Cruise) <roman.gushchin@linux.dev>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>