Messages in this thread | | | Date | Tue, 26 Sep 2017 13:35:22 -0500 | From | Reza Arbab <> | Subject | Re: [PATCH] mm/device-public-memory: Enable move_pages() to stat device memory |
| |
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
| |