memblock: improve MEMBLOCK_HOTPLUG documentation

The description of MEMBLOCK_HOTPLUG is currently short and consequently
misleading: we're actually dealing with a memory region that might get
hotunplugged later (i.e., the platform+firmware supports it), yet it is
indicated in the firmware-provided memory map as system ram that will
just get used by the system for any purpose when not taking special
care.  The firmware marked this memory region as a hot(un)plugged (e.g.,
hotplugged before reboot), implying that it might get hotunplugged again
later.

Whether we consider this information depends on the "movable_node"
kernel commandline parameter: only with "movable_node" set, we'll try
keeping this memory hotunpluggable, for example, by not serving early
allocations from this memory region and by letting the buddy manage it
using the ZONE_MOVABLE.

Let's make this clearer by extending the documentation.

Note: kexec *has to* indicate this memory to the second kernel.  With
"movable_node" set, we don't want to place kexec-images on this memory.
Without "movable_node" set, we don't care and can place kexec-images on
this memory.  In both cases, after successful memory hotunplug, kexec
has to be re-armed to update the memory map for the second kernel and to
place the kexec-images somewhere else.

Link: https://lkml.kernel.org/r/20211004093605.5830-3-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Cc: "Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Jianyong Wu <Jianyong.Wu@arm.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Shahab Vahedi <shahab@synopsys.com>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Vineet Gupta <vgupta@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/memblock.h

@@ -28,7 +28,11 @@ extern unsigned long long max_possible_pfn;
 /**
  * enum memblock_flags - definition of memory region attributes
  * @MEMBLOCK_NONE: no special request
- * @MEMBLOCK_HOTPLUG: hotpluggable region
+ * @MEMBLOCK_HOTPLUG: memory region indicated in the firmware-provided memory
+ * map during early boot as hot(un)pluggable system RAM (e.g., memory range
+ * that might get hotunplugged later). With "movable_node" set on the kernel
+ * commandline, try keeping this memory region hotunpluggable. Does not apply
+ * to memblocks added ("hotplugged") after early boot.
  * @MEMBLOCK_MIRROR: mirrored region
  * @MEMBLOCK_NOMAP: don't add to kernel direct mapping and treat as
  * reserved in the memory map; refer to memblock_mark_nomap() description