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

[Michael Ellerman]

Daniel Henrique Barboza <danielhb at linux.vnet.ibm.com> writes:

> 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?

> 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 at linux.vnet.ibm.com>
> ---
>  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

> 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