On Tue, Jan 26, 2016 at 2:11 PM, Andrew Morton
On Mon, 25 Jan 2016 16:06:40 -0800 Dan Williams
> 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?
I'm assuming at some point these legacy devices will die off or move
to something attached over a more capable bus like USB?
This was a pretty big goof - the removal of ZONE_DMA whizzed
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.
I endeavor to never surprise you again...
To be clear the patch did not disable ZONE_DMA by default, but it was
indeed a goof to assume that ZONE_DMA was less prevalent than it turns
out to be.
> We want a single
> kernel image to be able to support legacy devices as well as next
> generation persistent memory platforms.
> 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?
Last I tried I hit this warning in mm/memory.c
#warning Unfortunate NUMA and NUMA Balancing config, growing
page-frame for last_cpupid.
Also, this "dynamically added pfn of the zone" thing is a
and I think it should be more completely documented somewhere in the
Ok, I'll take a look.