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

[Reza Arbab]

On Tue, Sep 26, 2017 at 04:32:41PM +0000, Michal Hocko wrote:
> 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 don't know the definitive answer here, but as Jerome said PCIE devices 
should, and we are heading that way with NVLink/CAPI as well. It seems 
the default is just the nearest node.

> > 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.

As mentioned, it is minimally useful, e.g. for verifying migration, so 
returning the nid seems sensible to me. Alternatively, we might at least 
change the documentation to say 

-EFAULT
   This is a zero page, a device page, or the memory area is not mapped by the process.
			 ^^^^^^^^^^^^^

--
Reza Arbab