linux-kernel - Re: [HMM v14 05/16] mm/ZONE_DEVICE/unaddressable: add support for un-addressable device memory

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Date:   Thu, 8 Dec 2016 12:07:01 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, John Hubbard <jhubbard@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [HMM v14 05/16] mm/ZONE_DEVICE/unaddressable: add support for
 un-addressable device memory

On 12/08/2016 08:39 AM, Jerome Glisse wrote:
>> On 12/08/2016 08:39 AM, Jérôme Glisse wrote:
>>> > > Architecture that wish to support un-addressable device memory should make
>>> > > sure to never populate the kernel linar mapping for the physical range.
>> > 
>> > Does the platform somehow provide a range of physical addresses for this
>> > unaddressable area?  How do we know no memory will be hot-added in a
>> > range we're using for unaddressable device memory, for instance?
> That's what one of the big issue. No platform does not reserve any range so
> there is a possibility that some memory get hotpluged and assign this range.
> 
> I pushed the range decision to higher level (ie it is the device driver that
> pick one) so right now for device driver using HMM (NVidia close driver as
> we don't have nouveau ready for that yet) it goes from the highest physical
> address and scan down until finding an empty range big enough.

I don't think you should be stealing physical address space for things
that don't and can't have physical addresses.  Delegating this to
individual device drivers and hoping that they all get it right seems
like a recipe for disaster.

Maybe worth adding to the changelog:

	This feature potentially breaks memory hotplug unless every
	driver using it magically predicts the future addresses of
	where memory will be hotplugged.

BTW, how many more of these "big issues" does this set have?  I didn't
see any mention of this in the changelogs.