linux-kernel - Re: [PATCH v3] mm: add build-time option for hotplug memory default online type

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Message-ID: <5e958aaa-b1ac-4512-a592-0e1612032861@redhat.com>
Date: Sat, 21 Dec 2024 16:30:21 +0100
From: David Hildenbrand <david@...hat.com>
To: Gregory Price <gourry@...rry.net>, linux-mm@...ck.org
Cc: linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 loongarch@...ts.linux.dev, kernel-team@...a.com, corbet@....net,
 osalvador@...e.de, akpm@...ux-foundation.org, chenhuacai@...nel.org,
 kernel@...0n.name, gregkh@...uxfoundation.org, rafael@...nel.org
Subject: Re: [PATCH v3] mm: add build-time option for hotplug memory default
 online type


>   
> -config MEMORY_HOTPLUG_DEFAULT_ONLINE
> -	bool "Online the newly added memory blocks by default"
> -	depends on MEMORY_HOTPLUG
> +choice
> +	prompt "Memory Hotplug Default Online Type"
> +	default MHP_DEFAULT_ONLINE_TYPE_OFFLINE
>   	help
> +	  Default memory type for driver managed hotplug memory.

We should call it "hotplugged memory" consistently, which it is from a 
pure core-mm perspective ("add memory").

"Driver managed" reminds too much about add_memory_driver_managed(), 
which is only one case. Maybe just drop the "e.g., page tables" from the 
examples below.

> +
>   	  This option sets the default policy setting for memory hotplug
>   	  onlining policy (/sys/devices/system/memory/auto_online_blocks) which
>   	  determines what happens to newly added memory regions. Policy setting
>   	  can always be changed at runtime.
> +
> +	  The default is 'offline'.
> +
> +	  Select offline to defer onlining to drivers and user policy.
> +	  Select auto to let the kernel choose what zones to utilize.
> +	  Select online_kernel to generally allow kernel usage of this memory.
> +	  Select online_movable to generally disallow kernel usage of this memory.
> +
> +	  Example kernel usage would be page structs and page tables.
> +

>   	  See Documentation/admin-guide/mm/memory-hotplug.rst for more information.
>   
> -	  Say Y here if you want all hot-plugged memory blocks to appear in
> -	  'online' state by default.
> -	  Say N here if you want the default policy to keep all hot-plugged
> -	  memory blocks in 'offline' state.
> +config MHP_DEFAULT_ONLINE_TYPE_OFFLINE
> +	bool "offline"
> +	help
> +	  Driver managed memory will not be onlined by default.

"Hotplugged memory"

 > +	  Choose this for systems with drivers and user policy that> +	 
handle onlining of hotplug memory policy.
 > +> +config MHP_DEFAULT_ONLINE_TYPE_ONLINE_AUTO
> +	bool "auto"
> +	help
> +	  Select this if you want the kernel to automatically online
 > +	  memory into the zone it thinks is reasonable. This memory

hotplugged memory

> +	  may be utilized for kernel data (e.g. page tables).
> +
> +config MHP_DEFAULT_ONLINE_TYPE_ONLINE_KERNEL
> +	bool "kernel"
> +	help
> +	  Select this if you want the kernel to automatically online
> +	  hotplug memory into a zone capable of being used for kernel
> +	  data (e.g. page tables). This typically means ZONE_NORMAL.
> +
> +config MHP_DEFAULT_ONLINE_TYPE_ONLINE_MOVABLE
> +	bool "movable"
> +	help
> +	  Select this if you want the kernel to automatically online
> +	  hotplug memory into ZONE_MOVABLE. This memory will generally
> +	  not be utilized for kernel data (e.g. page tables).
> +
> +	  This should only be used when the admin knows sufficient
> +	  ZONE_NORMAL memory is available to describe hotplug memory,
> +	  otherwise hotplug memory may fail to online. For example,
> +	  sufficient kernel-capable memory (ZONE_NORMAL) must be
> +	  available to allocate page structs to describe ZONE_MOVABLE.
> +
> +endchoice
>   
>   config MEMORY_HOTREMOVE
>   	bool "Allow for memory hot remove"
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 3b6f93962481..e3655f07dd6e 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -219,11 +219,30 @@ void put_online_mems(void)
>   
>   bool movable_node_enabled = false;
>   
> -#ifndef CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE
> -int mhp_default_online_type = MMOP_OFFLINE;
> -#else
> -int mhp_default_online_type = MMOP_ONLINE;
> -#endif
> +static int mhp_default_online_type = -1;
> +int mhp_get_default_online_type(void)
> +{
> +	if (mhp_default_online_type >= 0)
> +		return mhp_default_online_type;
> +
> +	if (IS_ENABLED(CONFIG_MHP_DEFAULT_ONLINE_TYPE_OFFLINE))
> +		mhp_default_online_type = MMOP_OFFLINE;
> +	else if (IS_ENABLED(CONFIG_MHP_DEFAULT_ONLINE_TYPE_ONLINE_AUTO))
> +		mhp_default_online_type = MMOP_ONLINE;
> +	else if (IS_ENABLED(CONFIG_MHP_DEFAULT_ONLINE_TYPE_ONLINE_KERNEL))
> +		mhp_default_online_type = MMOP_ONLINE_KERNEL;
> +	else if (IS_ENABLED(CONFIG_MHP_DEFAULT_ONLINE_TYPE_ONLINE_MOVABLE))
> +		mhp_default_online_type = MMOP_ONLINE_MOVABLE;
> +	else
> +		mhp_default_online_type = MMOP_OFFLINE;

What would be nice is if we could use the symbols from Kconfig, to then 
only do

mhp_default_online_type = CONFIG_MHP_DEFAULT_ONLINE_TYPE;

But as far as I know, that's not possible.


Thanks!

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb