Re: [HMM 14/16] mm/hmm/devmem: device memory hotplug using ZONE_DEVICE

[Jerome Glisse]

On Fri, Apr 07, 2017 at 11:37:34AM +1000, Balbir Singh wrote:
> On Wed, 2017-04-05 at 16:40 -0400, Jérôme Glisse wrote:
> > This introduce a simple struct and associated helpers for device driver
> > to use when hotpluging un-addressable device memory as ZONE_DEVICE. It
> > will find a unuse physical address range and trigger memory hotplug for
> > it which allocates and initialize struct page for the device memory.
> > 
> > Signed-off-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
> > Signed-off-by: Evgeny Baskakov <ebaskakov@xxxxxxxxxx>
> > Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> > Signed-off-by: Mark Hairgrove <mhairgrove@xxxxxxxxxx>
> > Signed-off-by: Sherry Cheung <SCheung@xxxxxxxxxx>
> > Signed-off-by: Subhash Gutti <sgutti@xxxxxxxxxx>
> > ---
> >  include/linux/hmm.h | 114 +++++++++++++++
> >  mm/Kconfig          |   9 ++
> >  mm/hmm.c            | 398 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 521 insertions(+)
> > 
> > +/*
> > + * To add (hotplug) device memory, HMM assumes that there is no real resource
> > + * that reserves a range in the physical address space (this is intended to be
> > + * use by unaddressable device memory). It will reserve a physical range big
> > + * enough and allocate struct page for it.
> 
> I've found that the implementation of this is quite non-portable, in that
> starting from iomem_resource.end+1-size (which is effectively -size) on
> my platform (powerpc) does not give expected results. It could be that
> additional changes are needed to arch_add_memory() to support this
> use case.

The CDM version does not use that part, that being said isn't -size a valid
value we care only about unsigned here ? What is the end value on powerpc ?
In any case this sounds more like a unsigned/signed arithmetic issue, i will
look into it.

> 
> > +
> > +	size = ALIGN(size, SECTION_SIZE);
> > +	addr = (iomem_resource.end + 1ULL) - size;
> 
> 
> Why don't we allocate_resource() with the right constraints and get a new
> unused region?

The issue with allocate_resource() is that it does scan the resource tree
from lower address to higher ones. I was told that it was less likely to
have hotplug issue conflict if i pick highest physicall address for the
device memory hence why i do my own scan from the end toward the start.

Again all this function does not apply to PPC, it can be hidden behind
x86 config if you prefer it.

Cheers,
Jérôme