On Wed, Jul 01, 2020 at 02:07:44PM +0200, Daniel Vetter wrote: > Either my mailer ate half the thread or it's still stuck somewhere, so > jumping in the middle a bit. > > On Wed, Jul 01, 2020 at 11:03:06AM +0200, Christian König wrote: > > Am 30.06.20 um 20:46 schrieb Xiong, Jianxin: > > > > -----Original Message----- > > > > From: Jason Gunthorpe <jgg@xxxxxxxx> > > > > Sent: Tuesday, June 30, 2020 10:35 AM > > > > To: Xiong, Jianxin <jianxin.xiong@xxxxxxxxx> > > > > Cc: linux-rdma@xxxxxxxxxxxxxxx; Doug Ledford <dledford@xxxxxxxxxx>; Sumit Semwal <sumit.semwal@xxxxxxxxxx>; Leon Romanovsky > > > > <leon@xxxxxxxxxx>; Vetter, Daniel <daniel.vetter@xxxxxxxxx>; Christian Koenig <christian.koenig@xxxxxxx> > > > > Subject: Re: [RFC PATCH v2 0/3] RDMA: add dma-buf support > > > > > > > > On Tue, Jun 30, 2020 at 05:21:33PM +0000, Xiong, Jianxin wrote: > > > > > > > Heterogeneous Memory Management (HMM) utilizes > > > > > > > mmu_interval_notifier and ZONE_DEVICE to support shared virtual > > > > > > > address space and page migration between system memory and device > > > > > > > memory. HMM doesn't support pinning device memory because pages > > > > > > > located on device must be able to migrate to system memory when > > > > > > > accessed by CPU. Peer-to-peer access is possible if the peer can > > > > > > > handle page fault. For RDMA, that means the NIC must support on-demand paging. > > > > > > peer-peer access is currently not possible with hmm_range_fault(). > > > > > Currently hmm_range_fault() always sets the cpu access flag and device > > > > > private pages are migrated to the system RAM in the fault handler. > > > > > However, it's possible to have a modified code flow to keep the device > > > > > private page info for use with peer to peer access. > > > > Sort of, but only within the same device, RDMA or anything else generic can't reach inside a DEVICE_PRIVATE and extract anything useful. > > > But pfn is supposed to be all that is needed. > > > > > > > > > So.. this patch doesn't really do anything new? We could just make a MR against the DMA buf mmap and get to the same place? > > > > > That's right, the patch alone is just half of the story. The > > > > > functionality depends on availability of dma-buf exporter that can pin > > > > > the device memory. > > > > Well, what do you want to happen here? The RDMA parts are reasonable, but I don't want to add new functionality without a purpose - the > > > > other parts need to be settled out first. > > > At the RDMA side, we mainly want to check if the changes are acceptable. For example, > > > the part about adding 'fd' to the device ops and the ioctl interface. All the previous > > > comments are very helpful for us to refine the patch so that we can be ready when > > > GPU side support becomes available. > > > > > > > The need for the dynamic mapping support for even the current DMA Buf hacky P2P users is really too bad. Can you get any GPU driver to > > > > support non-dynamic mapping? > > > We are working on direct direction. > > > > > > > > > > migrate to system RAM. This is due to the lack of knowledge about > > > > > > > whether the importer can perform peer-to-peer access and the lack > > > > > > > of resource limit control measure for GPU. For the first part, the > > > > > > > latest dma-buf driver has a peer-to-peer flag for the importer, > > > > > > > but the flag is currently tied to dynamic mapping support, which > > > > > > > requires on-demand paging support from the NIC to work. > > > > > > ODP for DMA buf? > > > > > Right. > > > > Hum. This is not actually so hard to do. The whole dma buf proposal would make a lot more sense if the 'dma buf MR' had to be the > > > > dynamic kind and the driver had to provide the faulting. It would not be so hard to change mlx5 to be able to work like this, perhaps. (the > > > > locking might be a bit tricky though) > > > The main issue is that not all NICs support ODP. > > > > You don't need on-demand paging support from the NIC for dynamic mapping to > > work. > > > > All you need is the ability to stop wait for ongoing accesses to end and > > make sure that new ones grab a new mapping. > > So having no clue about rdma myself much, this sounds rather interesting. > Sure it would result in immediately re-acquiring the pages, but that's > also really all we need to be able to move buffers around on the gpu side. > And with dma_resv_lock there's no livelock risk if the NIC immediately > starts a kthread/work_struct which reacquires all the dma-buf and > everything else it needs. Plus also with the full ww_mutex deadlock > backoff dance there's no locking issues with having to acquire an entire > pile of dma_resv_lock, that's natively supported (gpus very much need to > be able to lock arbitrary set of buffers). > > And I think if that would allow us to avoid the entire "avoid random > drivers pinning dma-buf into vram" discussions, much better and quicker to > land something like that. > > I guess the big question is going to be how to fit this into rdma, since > the ww_mutex deadlock backoff dance needs to be done at a fairly high > level. For gpu drivers it's always done at the top level ioctl entry > point. Also, just to alleviate fears: I think all that dynamic dma-buf stuff for rdma should be doable this way _without_ having to interact with dma_fence. Avoiding that I think is the biggest request Jason has in this area :-) Furthermore, it is officially ok to allocate memory while holding a dma_resv_lock. What is not ok (and might cause issues if you somehow mix up things in strange ways) is taking a userspace fault, because gpu drivers must be able to take the dma_resv_lock in their fault handlers. That might pose a problem. Also, all these rules are now enforced by lockdep, might_fault() and similar checks. -Daniel > > > Apart from that this is a rather interesting work. > > > > Regards, > > Christian. > > > > > > > > > > > > There are a few possible ways to address these issues, such as > > > > > > > decoupling peer-to-peer flag from dynamic mapping, allowing more > > > > > > > leeway for individual drivers to make the pinning decision and > > > > > > > adding GPU resource limit control via cgroup. We would like to get > > > > > > > comments on this patch series with the assumption that device > > > > > > > memory pinning via dma-buf is supported by some GPU drivers, and > > > > > > > at the same time welcome open discussions on how to address the > > > > > > > aforementioned issues as well as GPU-NIC peer-to-peer access solutions in general. > > > > > > These seem like DMA buf problems, not RDMA problems, why are you > > > > > > asking these questions with a RDMA patch set? The usual DMA buf people are not even Cc'd here. > > > > > The intention is to have people from both RDMA and DMA buffer side to > > > > > comment. Sumit Semwal is the DMA buffer maintainer according to the > > > > > MAINTAINERS file. I agree more people could be invited to the discussion. > > > > > Just added Christian Koenig to the cc-list. > > > MAINTAINERS also says to cc and entire pile of mailing lists, where the > usual suspects (including Christian and me) hang around. Is that the > reason I got only like half the thread here? > > For next time around, really include everyone relevant here please. > -Daniel > > > > > Would be good to have added the drm lists too > > > Thanks, cc'd dri-devel here, and will also do the same for the previous part of the thread. > > > > > > > > If the umem_description you mentioned is for information used to > > > > > create the umem (e.g. a structure for all the parameters), then this would work better. > > > > It would make some more sense, and avoid all these weird EOPNOTSUPPS. > > > Good, thanks for the suggestion. > > > > > > > Jason > > > > _______________________________________________ > > dri-devel mailing list > > dri-devel@xxxxxxxxxxxxxxxxxxxxx > > https://lists.freedesktop.org/mailman/listinfo/dri-devel > > -- > Daniel Vetter > Software Engineer, Intel Corporation > http://blog.ffwll.ch -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
