On 08/01/2017 04:59 AM, Michael Ellerman wrote:

Daniel Henrique Barboza [off-list ref] writes:

quoted

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.

It's always polite to Cc the author of a commit you're referring to, so
I added Nathan.

The intention when we merged that fix was that the auto-online code
would be "fixed" to mark the device online. I say "fixed" because it
wasn't entirely clear if that was the correct behaviour, though it
definitely seemed like it should be.

I've lost track of where/if the discussion got to on whether the
auto-online code should do that or not. Did anything get resolved?

I think, though I should go back and test to be sure, that everything
works in the latest mainline code. The issue causing this to be a problem
was in the original implementation of auto_online support. If you wanted
to auto online memory, the code was calling memory_block_change_state().
This worked but did not update the device struct for each of the memory
block that was online'ed such that dev->offline == true even after the
memory was online.

I sent a patch earlier this year (commit dc18d706a436) that corrected
this to call device_online() instead of memory_block_change_state().
With this fix (appears to have gone into the 4.11 kernel) it should be
possible to use auto_online on power systems.

At this point I don't think we need this patch to disable auto online
for ppc64. I would be curious if this is still broken with the latest
mainline code though.

-Nathan
  

quoted

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 <redacted>
---
 mm/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

I don't own that file, so we at least need an Ack from the mm folks.

cheers

quoted

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