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

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Date:	Wed, 2 Mar 2016 09:10:53 +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 03/02/2016 12:43 AM, Dan Williams wrote:
> On Tue, Mar 1, 2016 at 12:31 AM, Vlastimil Babka <vbabka@...e.cz> wrote:
>> 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?
>
> So I went hunting for the x86_64 config that sent me off in this
> direction in the first place, but I can't reproduce it.  I'm indeed
> able to fit ZONE_DEVICE + ZONE_DMA + NODES_SHIFT(10) without
> overflowing page flags.  Maybe we reduced some usage page->flags usage
> between 4.3 and 4.5 and I missed it?

Oh, I think I see it now. SPARSEMEM_VMEMMAP_ENABLE only *allows to 
enable* CONFIG_SPARSEMEM_VMEMMAP, it doesn't force it:

config SPARSEMEM_VMEMMAP
         bool "Sparse Memory virtual memmap"
         depends on SPARSEMEM && SPARSEMEM_VMEMMAP_ENABLE
         default y

> In any event, you're right we can indeed fit ZONE_DEVICE into the
> current MAXSMP definition.  I'll respin the patch.

But I still believe that that your respin is better than this variant. 
We shouldn't broadly limit the range in one of the options, when there 
are multiple options affecting the usage of bits. There's a warning if 
the overal configuration is "too large", which could potentially be more 
detailed. But we never said configuring the kernel is trivial ;-)

Also in this case the "default y" for SPARSEMEM_VMEMMAP should prevent 
surprise when one enables ZONE_DEVICE through nvdimm and doesn't fiddle 
with the lowlevel details. As long as it takes multiple explicit choices 
differing from defaults to get to the warning, I'd say we are fine.

> Thanks for probing on this!
>