On Thu, Mar 23, 2023 at 11:42:07AM +0800, Jason Wang wrote:
On Tue, Mar 21, 2023 at 11:48 PM Stefano Garzarella <sgarzare@xxxxxxxxxx> wrote:
The new "use_va" module parameter (default: true) is used in
vdpa_alloc_device() to inform the vDPA framework that the device
supports VA.
vringh is initialized to use VA only when "use_va" is true and the
user's mm has been bound. So, only when the bus supports user VA
(e.g. vhost-vdpa).
vdpasim_mm_work_fn work is used to serialize the binding to a new
address space when the .bind_mm callback is invoked, and unbinding
when the .unbind_mm callback is invoked.
Call mmget_not_zero()/kthread_use_mm() inside the worker function
to pin the address space only as long as needed, following the
documentation of mmget() in include/linux/sched/mm.h:
* Never use this function to pin this address space for an
* unbounded/indefinite amount of time.
I wonder if everything would be simplified if we just allow the parent
to advertise whether or not it requires the address space.
Then when vhost-vDPA probes the device it can simply advertise
use_work as true so vhost core can use get_task_mm() in this case?
IIUC set user_worker to true, it also creates the kthread in the vhost
core (but we can add another variable to avoid this).
My biggest concern is the comment in include/linux/sched/mm.h.
get_task_mm() uses mmget(), but in the documentation they advise against
pinning the address space indefinitely, so I preferred in keeping
mmgrab() in the vhost core, then call mmget_not_zero() in the worker
only when it is running.
In the future maybe mm will be used differently from parent if somehow
it is supported by iommu, so I would leave it to the parent to handle
this.
Thanks,
Stefano
