Re: [PATCH V2] mm:memory hot-add: memory can not been added to movable zone

[Yasuaki Ishimatsu]

On Thu, 20 Aug 2015 03:28:05 -0400
Changsheng Liu <liuchangsheng@xxxxxxxxxx> wrote:

> From: Changsheng Liu <liuchangcheng@xxxxxxxxxx>
> 
> When memory is hot added, should_add_memory_movable() always returns 0
> because the movable zone is empty, so the memory that was hot added will
> add to the normal zone even if we want to remove the memory.
> 
> So we change should_add_memory_movable(): if the user config
> CONFIG_MOVABLE_NODE it will return 1 when the movable zone is empty.
> 
> Reviewed-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Changsheng Liu <liuchangcheng@xxxxxxxxxx>
> Tested-by: Dongdong Fan <fandd@xxxxxxxxxx>
> ---
>  mm/memory_hotplug.c |    3 +--
>  1 files changed, 1 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 26fbba7..ff658f2 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1199,8 +1199,7 @@ static int should_add_memory_movable(int nid, u64 start, u64 size)
>  	struct zone *movable_zone = pgdat->node_zones + ZONE_MOVABLE;
>  

>  	if (zone_is_empty(movable_zone))
> -		return 0;
> -
> +		return IS_ENABLED(CONFIG_MOVABLE_NODE);
>  	if (movable_zone->zone_start_pfn <= start_pfn)
>  		return 1;

Currently, kernel allows to create ZONE_MOVABLE after ZONE_NORMAL as follows:
 PFN low                                 high 
       ---|-------------|-------------|---
            ZONE_NORMAL   ZONE_MOVABLE

But kernel does not allow to create ZONE_MOVABLE before ZONE_NORMAL as follows:
 PFN low                                 high 
       ---|-------------|-------------|---
            ZONE_MOVABLE  ZONE_NORMAL

Also, kernel does not allow to create ZONE_MOVABLE in ZOME_NORMAL as follows:
 PFN low                                              high 
       ---|-------------|-------------|-------------|---
            ZONE_NORMAL   ZONE_MOVABLE  ZONE_NORMAL

So should_add_memory_movable() checks them.

Accoring to your patch, when movable_zone is empty, the hot added memory is
always managed to ZONE_MOVABLE. It means that ZONE_MOVALBE will puts before/in
ZONE_NORMAL.

You must prevent from creating ZONE_MOVABLE before/in ZONE_NORMAL.

Thanks,
Yasuaki Ishimatsu

>  
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