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

[Balbir Singh <bsingharora@gmail.com>]

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@redhat.com>
> Signed-off-by: Evgeny Baskakov <ebaskakov@nvidia.com>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> Signed-off-by: Mark Hairgrove <mhairgrove@nvidia.com>
> Signed-off-by: Sherry Cheung <SCheung@nvidia.com>
> Signed-off-by: Subhash Gutti <sgutti@nvidia.com>
> ---
>  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.

> +
> +	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?

Thanks,
Balbir