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

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Date:	Thu, 20 Aug 2015 09:41:59 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Changsheng Liu <liuchangsheng@...pur.com>,
	akpm@...ux-foundation.org, isimatu.yasuaki@...fujitsu.com
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	yanxiaofeng@...pur.com, fandd@...pur.com,
	Changsheng Liu <liuchangcheng@...pur.com>
Subject: Re: [PATCH V2] mm:memory hot-add: memory can not been added to
 movable zone

On 08/20/2015 09:28 AM, Changsheng Liu wrote:
> From: Changsheng Liu <liuchangcheng@...pur.com>
>
> 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.

I'm not expert on memory hot-plug, but since you CC'd me, I wonder... 
the function has this comment: " * If movable zone has already been 
setup, newly added memory should be check."

So I read it like "if you want movable memory *at all*, you should do 
some setup first" (but don't ask me what setup). After your patch, every 
hot-added memory would be automatically movable? Isn't that silently 
changing behavior against user expectations? What about those that don't 
want to hot-remove and don't want movable zones (which limit what kind 
of allocations are possible), is there a way to prevent memory being 
movable after your patch?

> 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@...ux-foundation.org>
> Signed-off-by: Changsheng Liu <liuchangcheng@...pur.com>
> Tested-by: Dongdong Fan <fandd@...pur.com>
> ---
>   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;
>
>

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