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Date: Fri, 14 Aug 2015 19:11:27 -0700
Message-ID: <CAPcyv4i-5RWTLK8FQFCBuFKwY0_HShbW7PVTHudSk4sF35xosA@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: Jerome Glisse <j.glisse@gmail.com>
Cc: "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>,
        Christoph Hellwig <hch@lst.de>
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On Fri, Aug 14, 2015 at 3:33 PM, Dan Williams <dan.j.williams@intel.com> wrote:
> On Fri, Aug 14, 2015 at 3:06 PM, Jerome Glisse <j.glisse@gmail.com> 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@gmail.com> 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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