Messages in this thread | | | Date | Thu, 23 Mar 2023 10:50:06 +0100 | From | Stefano Garzarella <> | Subject | Re: [PATCH v3 8/8] vdpa_sim: add support for user VA |
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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@redhat.com> 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
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