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

From: Dan Williams
Date: Fri Aug 14 2015 - 22:11:33 EST

On Fri, Aug 14, 2015 at 3:33 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> On Fri, Aug 14, 2015 at 3:06 PM, Jerome Glisse <j.glisse@xxxxxxxxx> wrote:
>> On Fri, Aug 14, 2015 at 02:52:15PM -0700, Dan Williams wrote:
>>> On Fri, Aug 14, 2015 at 2:37 PM, Jerome Glisse <j.glisse@xxxxxxxxx> wrote:
>>> > On Wed, Aug 12, 2015 at 11:50:05PM -0400, Dan Williams wrote:
> [..]
>>> > What is the rational for not updating max_pfn, max_low_pfn, ... ?
>>> >
>>> 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.
>> But this sounds bogus to me to rely on max_pfn to stay smaller than
>> first_dev_pfn. For instance you might plug a device that register
>> dev memory and then some regular memory might be hotplug, effectively
>> updating max_pfn to a value bigger than first_dev_pfn.
> True.
>> Also i do not think that the buddy allocator use max_pfn or max_low_pfn
>> to consider page/zone for allocation or not.
> Yes, I took it out with no effects. I'll investigate further whether
> we should be touching those variables or not for this new usage.

Although it does not offer perfect protection if device memory is at a
physically lower address than RAM, skipping the update of these
variables does seem to be what we want. For example /dev/mem would
fail to allow write access to persistent memory if it fails a
valid_phys_addr_range() check. Since /dev/mem does not know how to
write to PMEM in a reliably persistent way, it should not treat a
PMEM-pfn like RAM.
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