Radix vmemmap mapping can map things correctly at the PMD level or PTE
level based on different device boundary checks.  Hence we skip the
restrictions w.r.t vmemmap size to be multiple of PMD_SIZE.  This also
makes the feature widely useful because to use PMD_SIZE vmemmap area we
require a memory block size of 2GiB

We can also use MHP_RESERVE_PAGES_MEMMAP_ON_MEMORY to that the feature can
work with a memory block size of 256MB.  Using altmap.reserve feature to
align things correctly at pageblock granularity.  We can end up losing
some pages in memory with this.  For ex: with a 256MiB memory block size,
we require 4 pages to map vmemmap pages, In order to align things
correctly we end up adding a reserve of 28 pages.  ie, for every 4096
pages 28 pages get reserved.

Link: https://lkml.kernel.org/r/20230808091501.287660-6-aneesh.kumar@linux.ibm.comSigned-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>