[RFC PATCH] powerpc: Disabling MEMORY_HOTPLUG_DEFAULT_ONLINE option for PPC64 arch

[Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>]

Commit 943db62c316c ("powerpc/pseries: Revert 'Auto-online
hotplugged memory'") reverted the auto-online feature for pseries due
to problems with LMB removals not updating the device struct properly.
Among other things, this commit made the following change in
arch/powerpc/configs/pseries_defconfig:

@@ -58,7 +58,6 @@ CONFIG_KEXEC_FILE=y
 CONFIG_IRQ_ALL_CPUS=y
 CONFIG_MEMORY_HOTPLUG=y
 CONFIG_MEMORY_HOTREMOVE=y
-CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE=y
 CONFIG_KSM=y

The intent was to disable the option in the defconfig of pseries, since
after that the code doesn't have this support anymore. However, this change
alone isn't enough to prevent situations such as [1], where
distros can enable the option unaware of the consequences of
doing it (e.g. breaking LMB hotplug altogether).

Instead of relying on all distros knowing that pseries can't handle
CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE=y after 943db62c316c, this patch
changes mm/Kconfig to make the MEMORY_HOTPLUG_DEFAULT_ONLINE config
unavailable for the PPC64 arch.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1476380

Fixes: 943db62c316c ("powerpc/pseries: Revert 'Auto-online hotplugged memory'")
Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
---
 mm/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/Kconfig b/mm/Kconfig
index 48b1af4..a342c77 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -169,7 +169,7 @@ config MEMORY_HOTPLUG_SPARSE
 config MEMORY_HOTPLUG_DEFAULT_ONLINE
         bool "Online the newly added memory blocks by default"
         default n
-        depends on MEMORY_HOTPLUG
+        depends on MEMORY_HOTPLUG && !PPC64
         help
 	  This option sets the default policy setting for memory hotplug
 	  onlining policy (/sys/devices/system/memory/auto_online_blocks) which
-- 
2.9.4