linux-kernel - Re: [RFC PATCH] mm: CONFIG_NR_ZONES

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Date:	Tue, 1 Mar 2016 09:31:21 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux MM <linux-mm@...ck.org>, Mel Gorman <mgorman@...e.de>,
	Mark <markk@...ra.co.uk>, Joonsoo Kim <iamjoonsoo.kim@....com>,
	Sudip Mukherjee <sudipm.mukherjee@...il.com>
Subject: Re: [RFC PATCH] mm: CONFIG_NR_ZONES_EXTENDED

On 03/01/2016 03:06 AM, Dan Williams wrote:
> On Mon, Feb 29, 2016 at 4:06 PM, Vlastimil Babka <vbabka@...e.cz> wrote:
>> On 29.2.2016 18:55, Dan Williams wrote:
>>> On Mon, Feb 29, 2016 at 4:33 AM, Vlastimil Babka <vbabka@...e.cz> wrote:
>>>> On 02/02/2016 06:42 AM, Andrew Morton wrote:
>>>
>>> In this case it's already part of the equation because:
>>>
>>> config ZONE_DEVICE
>>>        depends on MEMORY_HOTPLUG
>>>        depends on MEMORY_HOTREMOVE
>>>
>>> ...and those in turn depend on SPARSEMEM.
>>
>> Fine, but then SPARSEMEM_VMEMMAP should be still an available subvariant of
>> SPARSEMEM with SECTION_WIDTH=0.
>
> It should be, but not for the ZONE_DEVICE case.  ZONE_DEVICE depends
> on x86_64 which means ZONE_DEVICE also implies SPARSEMEM_VMEMMAP
> since:
>
> config ARCH_SPARSEMEM_ENABLE
>         def_bool y
>         depends on X86_64 || NUMA || X86_32 || X86_32_NON_STANDARD
>         select SPARSEMEM_STATIC if X86_32
>         select SPARSEMEM_VMEMMAP_ENABLE if X86_64
>
> Now, if a future patch wants to reclaim page flags space for other
> usages outside of ZONE_DEVICE it can do the work to handle the
> SPARSEMEM_VMEMMAP=n case.  I don't see a reason to fold that
> distinction into the current patch given the current constraints.

OK so that IUUC shows that x86_64 should be always fine without decreasing the 
range for NODES_SHIFT? That's basically my point - since there's a configuration 
where things don't fit (32bit?), the patch broadly decreases range for 
NODES_SHIFT for everyone, right?

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