mm/memory_hotplug: support memmap_on_memory when memmap is not aligned to pageblocks

Currently, memmap_on_memory feature is only supported with memory block
sizes that result in vmemmap pages covering full page blocks.  This is
because memory onlining/offlining code requires applicable ranges to be
pageblock-aligned, for example, to set the migratetypes properly.

This patch helps to lift that restriction by reserving more pages than
required for vmemmap space.  This helps the start address to be page block
aligned with different memory block sizes.  Using this facility implies
the kernel will be reserving some pages for every memoryblock.  This
allows the memmap on memory feature to be widely useful with different
memory block size values.

For ex: with 64K page size and 256MiB memory block size, we require 4
pages to map vmemmap pages, 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/20230808091501.287660-5-aneesh.kumar@linux.ibm.com
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Michael Ellerman <mpe@ellerman.id.au>
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>

1 files changed, 12 insertions, 0 deletions

diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
index bd77841041af..2994958c7ce8 100644
--- a/Documentation/admin-guide/mm/memory-hotplug.rst
+++ b/Documentation/admin-guide/mm/memory-hotplug.rst
@@ -433,6 +433,18 @@ The following module parameters are currently defined:
 				 memory in a way that huge pages in bigger
 				 granularity cannot be formed on hotplugged
 				 memory.
+
+				 With value "force" it could result in memory
+				 wastage due to memmap size limitations. For
+				 example, if the memmap for a memory block
+				 requires 1 MiB, but the pageblock size is 2
+				 MiB, 1 MiB of hotplugged memory will be wasted.
+				 Note that there are still cases where the
+				 feature cannot be enforced: for example, if the
+				 memmap is smaller than a single page, or if the
+				 architecture does not support the forced mode
+				 in all configurations.
+
 ``online_policy``		 read-write: Set the basic policy used for
 				 automatic zone selection when onlining memory
 				 blocks without specifying a target zone.