mm/sparse: add vmemmap_*_hvo functions

mm/sparse: add vmemmap_*_hvo functions

Add a few functions to enable early HVO:

vmemmap_populate_hvo
vmemmap_undo_hvo
vmemmap_wrprotect_hvo

The populate and undo functions are expected to be used in early init,
from the sparse_init_nid_early() function.  The wrprotect function is to
be used, potentially, later.

To implement these functions, mostly re-use the existing compound pages
vmemmap logic used by DAX.  vmemmap_populate_address has its argument
changed a bit in this commit: the page structure passed in to be reused in
the mapping is replaced by a PFN and a flag.  The flag indicates whether
an extra ref should be taken on the vmemmap page containing the head page
structure.  Taking the ref is appropriate to for DAX / ZONE_DEVICE, but
not for HugeTLB HVO.

The HugeTLB vmemmap optimization maps tail page structure pages read-only.
The vmemmap_wrprotect_hvo function that does this is implemented
separately, because it cannot be guaranteed that reserved page structures
will not be write accessed during memory initialization.  Even with
CONFIG_DEFERRED_STRUCT_PAGE_INIT, they might still be written to (if they
are at the bottom of a zone).  So, vmemmap_populate_hvo leaves the tail
page structure pages RW initially, and then later during initialization,
after memmap init is fully done, vmemmap_wrprotect_hvo must be called to
finish the job.

Subsequent commits will use these functions for early HugeTLB HVO.

Link: https://lkml.kernel.org/r/20250228182928.2645936-15-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>