Re: [RFC PATCH 1/7] x86, mm: ZONE_DEVICE for "device memory"

[Dan Williams]

[ Adding David Woodhouse ]

On Sat, Aug 15, 2015 at 1:59 AM, Christoph Hellwig <hch@xxxxxx> wrote:
> On Fri, Aug 14, 2015 at 02:52:15PM -0700, Dan Williams wrote:
>> 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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