Re: [PATCH 1/6] mmput: use notifier chain to call subsystem exit handler.

[Joerg Roedel <joro-zLv9SwRftAIdnm+yROfE0A@public.gmane.org>]

On Tue, Jul 01, 2014 at 09:29:49AM +0000, Gabbay, Oded wrote:
> In the KFD, we need to maintain a notion of each compute process.
> Therefore, we have an object called "kfd_process" that is created for
> each process that uses the KFD. Naturally, we need to be able to track
> the process's shutdown in order to perform cleanup of the resources it
> uses (compute queues, virtual address space, gpu local memory
> allocations, etc.).

If it is only that, you can also use the task_exit notifier already in
the kernel.

> To enable this tracking mechanism, we decided to associate the
> kfd_process with mm_struct to ensure that a kfd_process object has
> exactly the same lifespan as the process it represents. We preferred to
> use the mm_struct and not a file description because using a file
> descriptor to track “process” shutdown is wrong in two ways:
> 
> * Technical: file descriptors can be passed to unrelated processes using
> AF_UNIX sockets. This means that a process can exit while the file stays
> open. Even if we implement this “correctly” i.e. holding the address
> space & page tables alive until the file is finally released, it’s
> really dodgy.

No, its not in this case. The file descriptor is used to connect a
process address space with a device context. Thus without the mappings
the file-descriptor is useless and the mappings should stay in-tact
until the fd is closed.

It would be a very bad semantic for userspace if a fd that is passed on
fails on the other side because the sending process died.


	Joerg


_______________________________________________
iommu mailing list
iommu@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/iommu