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

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Am 04.01.22 um 19:08 schrieb Felix Kuehling:
> [+Adrian]
>
> Am 2021-12-23 um 2:05 a.m. schrieb Christian König:
>
>> Am 22.12.21 um 21:53 schrieb Daniel Vetter:
>>> On Mon, Dec 20, 2021 at 01:12:51PM -0500, Bhardwaj, Rajneesh wrote:
>>>
>>> [SNIP]
>>> Still sounds funky. I think minimally we should have an ack from CRIU
>>> developers that this is officially the right way to solve this
>>> problem. I
>>> really don't want to have random one-off hacks that don't work across
>>> the
>>> board, for a problem where we (drm subsystem) really shouldn't be the
>>> only
>>> one with this problem. Where "this problem" means that the mmap space is
>>> per file description, and not per underlying inode or real device or
>>> whatever. That part sounds like a CRIU problem, and I expect CRIU folks
>>> want a consistent solution across the board for this. Hence please
>>> grab an
>>> ack from them.
>> Unfortunately it's a KFD design problem. AMD used a single device
>> node, then mmaped different objects from the same offset to different
>> processes and expected it to work the rest of the fs subsystem without
>> churn.
> This may be true for mmaps in the KFD device, but not for mmaps in the
> DRM render nodes.

Correct, yes.

>> So yes, this is indeed because the mmap space is per file descriptor
>> for the use case here.
> No. This is a different problem.

I was already wondering which mmaps through the KFD node we have left 
which cause problems here.

> The problem has to do with the way that DRM manages mmap permissions. In
> order to be able to mmap an offset in the render node, there needs to be
> a BO that was created in the same render node. If you fork a process, it
> inherits the VMA.

Yeah, so far it works like designed.

> But KFD doesn't know anything about the inherited BOs
> from the parent process.

Ok, why that? When the KFD is reinitializing it's context why shouldn't 
it cleanup those VMAs?

> Therefore those BOs don't get checkpointed and
> restored in the child process. When the CRIU checkpoint is restored, our
> CRIU plugin never creates a BO corresponding to the VMA in the child
> process' render node FD. We've also lost the relationship between the
> parent and child-process' render node FDs. After "fork" the render node
> FD points to the same struct file in parent and child. After restoring
> the CRIU checkpoint, they are separate struct files, created by separate
> "open" system calls. Therefore the mmap call that restores the VMA fails
> in the child process.
>
> At least for KFD, there is no point inheriting BOs from a child process,
> because the GPU has no way of accessing the BOs in the child process.
> The child process has no GPU address space, no user mode queues, no way
> to do anything with the GPU before it completely reinitializes its KFD
> context.
>
> We can workaround this issue in user mode with madvise(...,
> MADV_DONTFORK). In fact we've already done this for some BOs to avoid a
> memory leak in the parent process while a child process exists. But it's
> slightly racy because there is a short time window where VMA exists
> without the VM_DONTCOPY flag. A fork during that time window could still
> create a child process with an inherited VMA.
>
> Therefore a safer solution is to set the vm_flags in the VMA in the
> driver when the VMA is first created.

Thanks for the full explanation, it makes much more sense now.

Regards,
Christian.

>
> Regards,
>    Felix
>
>
>> And thanks for pointing this out, this indeed makes the whole change
>> extremely questionable.
>>
>> Regards,
>> Christian.
>>
>>> Cheers, Daniel
>>>