Re: [RFC PATCH 3/5] mm/vma: add support for peer to peer to device vma

[Jerome Glisse <jglisse@redhat.com>]

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
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