Re: [PATCH] mm/device-public-memory: Enable move_pages() to stat device memory

[Reza Arbab]

On Tue, Sep 26, 2017 at 01:37:07PM +0000, Michal Hocko wrote:
> On Fri 22-09-17 15:13:56, Reza Arbab wrote:
> > The move_pages() syscall can be used to find the numa node where a page
> > currently resides. This is not working for device public memory pages,
> > which erroneously report -EFAULT (unmapped or zero page).
> >
> > Enable by adding a FOLL_DEVICE flag for follow_page(), which
> > move_pages() will use. This could be done unconditionally, but adding a
> > flag seems like a safer change.
>
> I do not understand purpose of this patch. What is the numa node of a
> device memory?

Well, using hmm_devmem_pages_create() it is added to this node:

	nid = dev_to_node(device);
	if (nid < 0)
		nid = numa_mem_id();

I understand it's minimally useful information to userspace, but the 
memory does have a nid and move_pages() is supposed to be able to return 
what that is. I ran into this using a testcase which tries to verify 
that user addresses were correctly migrated to coherent device memory.

That said, I'm okay with dropping this if you don't think it's 
worthwhile.

--
Reza Arbab