[193/262] memblock: improve MEMBLOCK_HOTPLUG documentation

Commit Message

Andrew Morton Nov. 5, 2021, 8:44 p.m. UTC

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

 include/linux/memblock.h |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

Patch

--- a/include/linux/memblock.h~memblock-improve-memblock_hotplug-documentation
+++ a/include/linux/memblock.h
@@ -28,7 +28,11 @@  extern unsigned long long max_possible_p
 /**
  * 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