Re: [Linaro-mm-sig] Changing vma->vm_file in dma_buf_mmap()

From: Christian König
Date: Thu Sep 17 2020 - 12:26:01 EST

Am 17.09.20 um 17:37 schrieb Daniel Vetter:
On Thu, Sep 17, 2020 at 5:24 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
On Thu, Sep 17, 2020 at 04:54:44PM +0200, Christian König wrote:
Am 17.09.20 um 16:35 schrieb Jason Gunthorpe:
On Thu, Sep 17, 2020 at 02:24:29PM +0200, Christian König wrote:
Am 17.09.20 um 14:18 schrieb Jason Gunthorpe:
On Thu, Sep 17, 2020 at 02:03:48PM +0200, Christian König wrote:
Am 17.09.20 um 13:31 schrieb Jason Gunthorpe:
On Thu, Sep 17, 2020 at 10:09:12AM +0200, Daniel Vetter wrote:

Yeah, but it doesn't work when forwarding from the drm chardev to the
dma-buf on the importer side, since you'd need a ton of different
address spaces. And you still rely on the core code picking up your
pgoff mangling, which feels about as risky to me as the vma file
pointer wrangling - if it's not consistently applied the reverse map
is toast and unmap_mapping_range doesn't work correctly for our needs.
I would think the pgoff has to be translated at the same time the
vm->vm_file is changed?

The owner of the dma_buf should have one virtual address space and FD,
all its dma bufs should be linked to it, and all pgoffs translated to
that space.
Yeah, that is exactly like amdgpu is doing it.

Going to document that somehow when I'm done with TTM cleanups.
BTW, while people are looking at this, is there a way to go from a VMA
to a dma_buf that owns it?
Only a driver specific one.
Sounds OK

For TTM drivers vma->vm_private_data points to the buffer object. Not sure
about the drivers using GEM only.
Why are drivers in control of the vma? I would think dma_buf should be
the vma owner. IIRC module lifetime correctness essentially hings on
the module owner of the struct file
Because the page fault handling is completely driver specific.

We could install some DMA-buf vmops, but that would just be another layer of
Uh geez I didn't know amdgpu was doing that :-/

Since this is on, I guess the inverse of trying to convert a userptr
into a dma-buf is properly rejected?

My fault, I wasn't specific enough in my description :)

Amdgpu is NOT doing this with mmaped DMA-bufs, but rather with it's own mmaped BOs.

In other words when userspace call the userptr IOCTL and we get an error because we can't make an userptr from some random device memory we instead check all CPU mappings if the application was brain dead enough to provide us our own pointer back.

IIRC this is even done in userspace and not the kernel. But we talked about doing it in the kernel with the private_data as well.

If it is already taking a page fault I'm not sure the extra function
call indirection is going to be a big deal. Having a uniform VMA
sounds saner than every driver custom rolling something.

When I unwound a similar mess in RDMA all the custom VMA stuff in the
drivers turned out to be generally buggy, at least.

Is vma->vm_file->private_data universally a dma_buf pointer at least?
Nope. I think if you want this without some large scale rewrite of a
lot of code we'd need a vmops->get_dmabuf or similar. Not pretty, but
would get the job done.

Yeah, agree that sounds like the simplest approach.


So, user VA -> find_vma -> dma_buf object -> dma_buf operations on the
memory it represents
Ah, yes we are already doing this in amdgpu as well. But only for DMA-bufs
or more generally buffers which are mmaped by this driver instance.
So there is no general dma_buf service? That is a real bummer
Mostly historical reasons and "it's complicated". One problem is that
dma-buf isn't a powerful enough interface that drivers could use it
for all their native objects, e.g. userptr doesn't pass through it,
and clever cache flushing tricks aren't allowed and a bunch of other
things. So there's some serious roadblocks before we could have a
common allocator (or set of allocators) behind dma-buf.