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

[Michal Hocko]

On Tue 26-09-17 09:47:10, Reza Arbab wrote:
> 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();

OK, but do all the HMM devices have concept of NUMA affinity? From the
code you are pasting they do not have to...
 
> 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.

I am just worried that we allow information which is not generally
sensible and I am also not sure what the userspace can actually do with
that information.
-- 
Michal Hocko
SUSE Labs