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

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Date:	Tue, 26 Jan 2016 14:11:52 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Rik van Riel <riel@...hat.com>, linux-nvdimm@...1.01.org,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
	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 Mon, 25 Jan 2016 16:06:40 -0800 Dan Williams <dan.j.williams@...el.com> wrote:

> It appears devices requiring ZONE_DMA are still prevalent (see link
> below).  For this reason the proposal to require turning off ZONE_DMA to
> enable ZONE_DEVICE is untenable in the short term.

More than "short term".  When can we ever nuke ZONE_DMA?

This was a pretty big goof - the removal of ZONE_DMA whizzed straight
past my attention, alas.  In fact I never noticed the patch at all
until I got some conflicts in -next a few weeks later (wasn't cc'ed). 
And then I didn't read the changelog closely enough.

>  We want a single
> kernel image to be able to support legacy devices as well as next
> generation persistent memory platforms.

yup.
 
> Towards this end, alias ZONE_DMA and ZONE_DEVICE to work around needing
> to maintain a unique zone number for ZONE_DEVICE.  Record the geometry
> of ZONE_DMA at init (->init_spanned_pages) and use that information in
> is_zone_device_page() to differentiate pages allocated via
> devm_memremap_pages() vs true ZONE_DMA pages.  Otherwise, use the
> simpler definition of is_zone_device_page() when ZONE_DMA is turned off.
> 
> Note that this also teaches the memory hot remove path that the zone may
> not have sections for all pfn spans (->zone_dyn_start_pfn).
> 
> A user visible implication of this change is potentially an unexpectedly
> high "spanned" value in /proc/zoneinfo for the DMA zone.

Well, all these icky tricks are to avoid increasing ZONES_SHIFT, yes? 
Is it possible to just use ZONES_SHIFT=3?

Also, this "dynamically added pfn of the zone" thing is a new concept
and I think it should be more completely documented somewhere in the
code.