[PATCH] drm/ttm: Don't inherit GEM object VMAs in child process

[Marek Olšák]

I don't think fork() would work with userspace where all buffers are
shared. It certainly doesn't work now. The driver needs to be notified that
a buffer or texture is shared to ensure data coherency between processes,
and the driver must execute decompression and other render passes when a
buffer or texture is being shared for the first time. Those aren't called
when fork() is called.

Marek

On Mon, Jan 17, 2022 at 9:34 AM Felix Kuehling <felix.kuehling at amd.com>
wrote:

> Am 2022-01-17 um 9:21 a.m. schrieb Christian König:
> > Am 17.01.22 um 15:17 schrieb Felix Kuehling:
> >> Am 2022-01-17 um 6:44 a.m. schrieb Christian König:
> >>> Am 14.01.22 um 18:40 schrieb Felix Kuehling:
> >>>> Am 2022-01-14 um 12:26 p.m. schrieb Christian König:
> >>>>> Am 14.01.22 um 17:44 schrieb Daniel Vetter:
> >>>>>> Top post because I tried to catch up on the entire discussion here.
> >>>>>>
> >>>>>> So fundamentally I'm not opposed to just close this fork() hole
> >>>>>> once and
> >>>>>> for all. The thing that worries me from a upstream/platform pov is
> >>>>>> really
> >>>>>> only if we don't do it consistently across all drivers.
> >>>>>>
> >>>>>> So maybe as an idea:
> >>>>>> - Do the original patch, but not just for ttm but all gem rendernode
> >>>>>>      drivers at least (or maybe even all gem drivers, no idea), with
> >>>>>> the
> >>>>>>      below discussion cleaned up as justification.
> >>>>> I know of at least one use case which this will break.
> >>>>>
> >>>>> A couple of years back we had a discussion on the Mesa mailing list
> >>>>> because (IIRC) Marek introduced a background thread to push command
> >>>>> submissions to the kernel.
> >>>>>
> >>>>> That broke because some compositor used to initialize OpenGL and then
> >>>>> do a fork(). This indeed worked previously (no GPUVM at that time),
> >>>>> but with the addition of the backround thread obviously broke.
> >>>>>
> >>>>> The conclusion back then was that the compositor is broken and needs
> >>>>> fixing, but it still essentially means that there could be people out
> >>>>> there with really old userspace where this setting would just break
> >>>>> the desktop.
> >>>>>
> >>>>> I'm not really against that change either, but at least in theory we
> >>>>> could make fork() work perfectly fine even with VMs and background
> >>>>> threads.
> >>>> You may regret this if you ever try to build a shared virtual address
> >>>> space between GPU and CPU. Then you have two processes (parent and
> >>>> child) sharing the same render context and GPU VM address space.
> >>>> But the
> >>>> CPU address spaces are different. You can't maintain consistent shared
> >>>> virtual address spaces for both processes when the GPU address
> >>>> space is
> >>>> shared between them.
> >>> That's actually not much of a problem.
> >>>
> >>> All you need to do is to use pthread_atfork() and do the appropriate
> >>> action in parent/child to clean up your context:
> >>> https://man7.org/linux/man-pages/man3/pthread_atfork.3.html
> >> Thunk already does that. However, it's not foolproof. pthread_atfork
> >> hanlders aren't called when the process is forked with a clone call.
> >
> > Yeah, but that's perfectly intentional. clone() is usually used to
> > create threads.
>
> Clone can be used to create new processes. Maybe not the common use today.
>
>
> >
> >>> The rest is just to make sure that all shared and all private data are
> >>> kept separate all the time. Sharing virtual memory is already done for
> >>> decades this way, it's just that nobody ever did it with a statefull
> >>> device like GPUs.
> >> My concern is not with sharing or not sharing data. It's with sharing
> >> the address space itself. If you share the render node, you share GPU
> >> virtual address space. However CPU address space is not shared between
> >> parent and child. That's a fundamental mismatch between the CPU world
> >> and current GPU driver implementation.
> >
> > Correct, but even that is easily solvable. As I said before you can
> > hang this state on a VMA and let it be cloned together with the CPU
> > address space.
>
> I'm not following. The address space I'm talking about is struct
> amdgpu_vm. It's associated with the render node file descriptor.
> Inheriting and using that file descriptor in the child inherits the
> amdgpu_vm. I don't see how you can hang that state on any one VMA.
>
> To be consistent with the CPU, you'd need to clone the GPU address space
> (struct amdgpu_vm) in the child process. That means you need a new
> render node file descriptor that imports all the BOs from the parent
> address space. It's a bunch of extra work to fork a process, that you're
> proposing to immediately undo with an atfork handler. So I really don't
> see the point.
>
> Regards,
>   Felix
>
>
> >
> > Since VMAs are informed about their cloning (in opposite to file
> > descriptors) it's trivial to even just clone kernel data on first access.
> >
> > Regards,
> > Christian.
> >
> >>
> >> Regards,
> >>    Felix
> >>
> >>
> >>> Regards,
> >>> Christian.
> >>>
> >>>> Regards,
> >>>>     Felix
> >>>>
> >
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.freedesktop.org/archives/amd-gfx/attachments/20220117/87f7b598/attachment.htm>