[ Adding David Woodhouse ]

On Sat, Aug 15, 2015 at 1:59 AM, Christoph Hellwig [off-list ref] wrote:

On Fri, Aug 14, 2015 at 02:52:15PM -0700, Dan Williams wrote:

quoted

The idea is that this memory is not meant to be available to the page
allocator and should not count as new memory capacity.  We're only
hotplugging it to get struct page coverage.

This might need a bigger audit of the max_pfn usages.  I remember
architectures using it as a decisions for using IOMMUs or similar.

We chatted about this at LPC yesterday.  The takeaway was that the
max_pfn checks that the IOMMU code does is for checking whether a
device needs an io-virtual mapping to reach addresses above its DMA
limit (if it can't do 64-bit DMA).  Given the capacities of persistent
memory it's likely that a device with this limitation already can't
address all of RAM let alone PMEM.   So it seems to me that updating
max_pfn for PMEM hotplug does not buy us anything except a few more
opportunities to confuse PMEM as typical RAM.

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