From: Dan Williams
Date: Tue Jan 26 2016 - 12:07:12 EST

On Mon, Jan 25, 2016 at 10:00 PM, Sudip Mukherjee
<sudipm.mukherjee@xxxxxxxxx> wrote:
> On Mon, Jan 25, 2016 at 04:06:40PM -0800, Dan Williams 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. 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.
>> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: Rik van Riel <riel@xxxxxxxxxx>
>> Cc: Mel Gorman <mgorman@xxxxxxx>
>> Cc: Jerome Glisse <j.glisse@xxxxxxxxx>
>> Cc: Christoph Hellwig <hch@xxxxxx>
>> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>> Link:
>> Fixes: 033fbae988fc ("mm: ZONE_DEVICE for "device memory"")
>> Reported-by: Sudip Mukherjee <sudipm.mukherjee@xxxxxxxxx>
> It should actually be Reported-by: Mark <markk@xxxxxxxxxxx>
> Hi Mark,
> Can you please test this patch available at
> in your setup..

Note this patch is on top of 4.5-rc1 and is likely not a suitable for
-stable backport to 4.3/4.4. For 4.3 and 4.4, distributions that want
to support legacy devices should leave ZONE_DEVICE disabled as it is
by default.