linux-kernel - Re: [RFC PATCH 1/7] x86, mm: ZONE

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Message-ID: <CAPcyv4i-5RWTLK8FQFCBuFKwY0_HShbW7PVTHudSk4sF35xosA@mail.gmail.com>
Date:	Fri, 14 Aug 2015 19:11:27 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Jerome Glisse <j.glisse@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Boaz Harrosh <boaz@...xistor.com>,
	Rik van Riel <riel@...hat.com>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	david <david@...morbit.com>, Ingo Molnar <mingo@...nel.org>,
	Linux MM <linux-mm@...ck.org>, Ingo Molnar <mingo@...hat.com>,
	Mel Gorman <mgorman@...e.de>, "H. Peter Anvin" <hpa@...or.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	Christoph Hellwig <hch@....de>
Subject: Re: [RFC PATCH 1/7] x86, mm: ZONE_DEVICE for "device memory"

On Fri, Aug 14, 2015 at 3:33 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Fri, Aug 14, 2015 at 3:06 PM, Jerome Glisse <j.glisse@...il.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@...il.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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