linux-kernel - Re: [RFC PATCH] mm: support CONFIG_ZONE_DEVICE + CONFIG_ZONE

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Date:	Tue, 26 Jan 2016 19:23:59 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	linux-nvdimm <linux-nvdimm@...1.01.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>, Linux MM <linux-mm@...ck.org>,
	Ingo Molnar <mingo@...hat.com>, Mel Gorman <mgorman@...e.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jerome Glisse <j.glisse@...il.com>,
	Sudip Mukherjee <sudipm.mukherjee@...il.com>
Subject: Re: [RFC PATCH] mm: support CONFIG_ZONE_DEVICE + CONFIG_ZONE_DMA

On Tue, Jan 26, 2016 at 6:15 PM, Joonsoo Kim <iamjoonsoo.kim@....com> wrote:
> On Tue, Jan 26, 2016 at 05:37:38PM -0800, Dan Williams wrote:
>> On Tue, Jan 26, 2016 at 5:18 PM, Joonsoo Kim <iamjoonsoo.kim@....com> wrote:
[..]
>> > Please refer my previous attempt to add a new zone, ZONE_CMA.
>> >
>> > https://lkml.org/lkml/2015/2/12/84
>> >
>> > It salvages a bit from SECTION_WIDTH by increasing section size.
>> > Similarly, I guess we can reduce NODE_WIDTH if needed although
>> > it could cause to reduce maximum node size.
>>
>> Dave pointed out to me that LAST__PID_SHIFT might be a better
>> candidate to reduce to 7 bits.  That field is for storing pids which
>> are already bigger than 8 bits.  If it is relying on the fact that
>> pids don't rollover very often then likely the impact of 7-bits
>> instead of 8 will be minimal.
>
> Hmm... I'm not sure it's possible or not, but, it looks not a general
> solution. It will solve your problem because you are using 64 bit arch
> but other 32 bit archs can't get the benefit.

This is where the ZONE_CMA and ZONE_DEVICE efforts diverge.
ZONE_DEVICE is meant to enable DMA access to hundreds of gigagbytes of
persistent memory.  A 64-bit-only limitation for ZONE_DEVICE is
reasonable.