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/[email protected]
Signed-off-by: David Hildenbrand <[email protected]>
Reviewed-by: Mike Rapoport <[email protected]>
Cc: "Aneesh Kumar K . V" <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Eric Biederman <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Huacai Chen <[email protected]>
Cc: Jianyong Wu <[email protected]>
Cc: Jiaxun Yang <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Shahab Vahedi <[email protected]>
Cc: Thomas Bogendoerfer <[email protected]>
Cc: Vasily Gorbik <[email protected]>
Cc: Vineet Gupta <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 5 insertions, 1 deletions

diff --git a/include/linux/memblock.h b/include/linux/memblock.h
index 484650681bee..1d2b3e902a84 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