linux-kernel - Re: [PATCH] mm: ZONE_DEVICE depends on SPARSEMEM

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Date:	Mon, 7 Mar 2016 10:53:35 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Dan Williams <dan.j.williams@...el.com>, akpm@...ux-foundation.org
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: ZONE_DEVICE depends on SPARSEMEM_VMEMMAP

On 03/05/2016 01:42 AM, Dan Williams wrote:
> The primary use case for devm_memremap_pages() is to allocate an
> memmap array from persistent memory.  That capabilty requires
> vmem_altmap which requires SPARSEMEM_VMEMMAP.

Some if the ifdefs around to_vmem_altmap() could now be simplified, but 
that can also be done later...

> Also, without SPARSEMEM_VMEMMAP the addition of ZONE_DEVICE expands
> ZONES_WIDTH and triggers the:
>
> "Unfortunate NUMA and NUMA Balancing config, growing page-frame for
> last_cpupid."
>
> ...warning in mm/memory.c.  SPARSEMEM_VMEMMAP=n && ZONE_DEVICE=y is not
> a configuration we should worry about supporting.
>
> Reported-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>   mm/Kconfig |    1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 664fa2416909..b95322ba542b 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -654,6 +654,7 @@ config ZONE_DEVICE
>   	bool "Device memory (pmem, etc...) hotplug support" if EXPERT
>   	depends on MEMORY_HOTPLUG
>   	depends on MEMORY_HOTREMOVE
> +	depends on SPARSEMEM_VMEMMAP
>   	depends on X86_64 #arch_add_memory() comprehends device memory
>
>   	help
>