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SubjectRe: [RFC PATCH] mm: support CONFIG_ZONE_DEVICE + CONFIG_ZONE_DMA
On Mon, 25 Jan 2016 16:06:40 -0800 Dan Williams <dan.j.williams@intel.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.

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