On Tue 25-06-19 20:15:28, John Hubbard wrote: > On 6/19/19 12:27 PM, Jason Gunthorpe wrote: > > On Thu, Jun 13, 2019 at 06:23:04PM -0700, John Hubbard wrote: > >> On 6/13/19 5:43 PM, Ira Weiny wrote: > >>> On Thu, Jun 13, 2019 at 07:58:29PM +0000, Jason Gunthorpe wrote: > >>>> On Thu, Jun 13, 2019 at 12:53:02PM -0700, Ralph Campbell wrote: > >>>>> > >> ... > >>> So I think it is ok. Frankly I was wondering if we should remove the public > >>> type altogether but conceptually it seems ok. But I don't see any users of it > >>> so... should we get rid of it in the code rather than turning the config off? > >>> > >>> Ira > >> > >> That seems reasonable. I recall that the hope was for those IBM Power 9 > >> systems to use _PUBLIC, as they have hardware-based coherent device (GPU) > >> memory, and so the memory really is visible to the CPU. And the IBM team > >> was thinking of taking advantage of it. But I haven't seen anything on > >> that front for a while. > > > > Does anyone know who those people are and can we encourage them to > > send some patches? :) > > > > I asked about this, and it seems that the idea was: DEVICE_PUBLIC was there > in order to provide an alternative way to do things (such as migrate memory > to and from a device), in case the combination of existing and near-future > NUMA APIs was insufficient. This probably came as a follow-up to the early > 2017-ish conversations about NUMA, in which the linux-mm recommendation was > "try using HMM mechanisms, and if those are inadequate, then maybe we can > look at enhancing NUMA so that it has better handling of advanced (GPU-like) > devices". Yes that was the original idea. It sounds so much better to use a common framework rather than awkward special cased cpuless NUMA nodes with a weird semantic. User of the neither of the two has shown up so I guess that the envisioned HW just didn't materialized. Or has there been a completely different approach chosen? > In the end, however, _PUBLIC was never used, nor does anyone in the local > (NVIDIA + IBM) kernel vicinity seem to have plans to use it. So it really > does seem safe to remove, although of course it's good to start with > BROKEN and see if anyone pops up and complains. Well, I do not really see much of a difference. Preserving an unused code which doesn't have any user in sight just adds a maintenance burden whether the code depends on BROKEN or not. We can always revert patches which remove the code once a real user shows up. -- Michal Hocko SUSE Labs
