Allocate memory map (struct pages array) from the hotplugged memory
range, rather than using system memory. The change addresses the issue
where standby memory, when configured to be much larger than online
memory, could potentially lead to ipl failure due to memory map
allocation from online memory. For example, 16MB of memory map
allocation is needed for a memory block size of 1GB and when standby
memory is configured much larger than online memory, this could lead to
ipl failure.

To address this issue, the solution involves introducing "memmap on
memory" using the vmem_altmap structure on s390.  Architectures that
want to implement it should pass the altmap to the vmemmap_populate()
function and its associated callchain. This enhancement is discussed in
commit 4b94ffdc ("x86, mm: introduce vmem_altmap to augment
vmemmap_populate()")

Provide "memmap on memory" support for s390 by passing the altmap in
vmemmap_populate() and its callchain. The allocation path is described
as follows:
* When altmap is NULL in vmemmap_populate(), memory map allocation
  occurs using the existing vmemmap_alloc_block_buf().
* When altmap is not NULL in vmemmap_populate(), memory map allocation
  still uses vmemmap_alloc_block_buf(), but this function internally
  calls altmap_alloc_block_buf().

For deallocation, the process is outlined as follows:
* When altmap is NULL in vmemmap_free(), memory map deallocation happens
  through free_pages().
* When altmap is not NULL in vmemmap_free(), memory map deallocation
  occurs via vmem_altmap_free().

While memory map allocation is primarily handled through the
self-contained memory map range, there might still be a small amount of
system memory allocation required for vmemmap pagetables. To mitigate
this impact, this feature will be limited to machines with EDAT1
support.

Link: https://lkml.kernel.org/r/20240108132747.3238763-3-sumanthk@linux.ibm.comReviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>