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Date: Fri, 21 Aug 2015 08:02:51 -0700
Message-ID: <CAPcyv4jiXqcy_kUrArw7cpbySDoaQe+UF5JcvihxyGyxjsWKZw@mail.gmail.com>
Subject: Re: [RFC PATCH 1/7] x86, mm: ZONE_DEVICE for "device memory"
From: Dan Williams <dan.j.williams@intel.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Jerome Glisse <j.glisse@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Boaz Harrosh <boaz@plexistor.com>, Rik van Riel <riel@redhat.com>,
        "linux-nvdimm@lists.01.org" <linux-nvdimm@ml01.01.org>,
        Dave Hansen <dave.hansen@linux.intel.com>, david <david@fromorbit.com>,
        Ingo Molnar <mingo@kernel.org>, Linux MM <linux-mm@kvack.org>,
        Ingo Molnar <mingo@redhat.com>, Mel Gorman <mgorman@suse.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
        "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
        David Woodhouse <dwmw2@infradead.org>
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[ Adding David Woodhouse ]

On Sat, Aug 15, 2015 at 1:59 AM, Christoph Hellwig <hch@lst.de> 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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