On Wed, Jan 30, 2019 at 08:50:00PM +0000, Jason Gunthorpe wrote: > On Wed, Jan 30, 2019 at 03:43:32PM -0500, Jerome Glisse wrote: > > On Wed, Jan 30, 2019 at 08:11:19PM +0000, Jason Gunthorpe wrote: > > > On Wed, Jan 30, 2019 at 01:00:02PM -0700, Logan Gunthorpe wrote: > > > > > > > We never changed SGLs. We still use them to pass p2pdma pages, only we > > > > need to be a bit careful where we send the entire SGL. I see no reason > > > > why we can't continue to be careful once their in userspace if there's > > > > something in GUP to deny them. > > > > > > > > It would be nice to have heterogeneous SGLs and it is something we > > > > should work toward but in practice they aren't really necessary at the > > > > moment. > > > > > > RDMA generally cannot cope well with an API that requires homogeneous > > > SGLs.. User space can construct complex MRs (particularly with the > > > proposed SGL MR flow) and we must marshal that into a single SGL or > > > the drivers fall apart. > > > > > > Jerome explained that GPU is worse, a single VMA may have a random mix > > > of CPU or device pages.. > > > > > > This is a pretty big blocker that would have to somehow be fixed. > > > > Note that HMM takes care of that RDMA ODP with my ODP to HMM patch, > > so what you get for an ODP umem is just a list of dma address you > > can program your device to. The aim is to avoid the driver to care > > about that. The access policy when the UMEM object is created by > > userspace through verbs API should however ascertain that for mmap > > of device file it is only creating a UMEM that is fully covered by > > one and only one vma. GPU device driver will have one vma per logical > > GPU object. I expect other kind of device do that same so that they > > can match a vma to a unique object in their driver. > > A one VMA rule is not really workable. > > With ODP VMA boundaries can move around across the lifetime of the MR > and we have no obvious way to fail anything if userpace puts a VMA > boundary in the middle of an existing ODP MR address range. This is true only for vma that are not mmap of a device file. This is what i was trying to get accross. An mmap of a file is never merge so it can only get split/butcher by munmap/mremap but when that happen you also need to reflect the virtual address space change to the device ie any access to a now invalid range must trigger error. > > I think the HMM mirror API really needs to deal with this for the > driver somehow. Yes the HMM does deal with this for you, you do not have to worry about it. Sorry if that was not clear. I just wanted to stress that vma that are mmap of a file do not behave like other vma hence when you create the UMEM you can check for those if you feel the need. Cheers, Jérôme
