[PATCH v2 2/5] memblock: improve MEMBLOCK_HOTPLUG documentation

[David Hildenbrand]

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.

Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 include/linux/memblock.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/linux/memblock.h b/include/linux/memblock.h
index 34de69b3b8ba..4ee8dd2d63a7 100644
--- a/include/linux/memblock.h
+++ b/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
-- 
2.31.1