vhost: use mmgrab() instead of mmget() for non worker device
For the device that doesn't use vhost worker and use_mm(), mmget() is too heavy weight and it may brings troubles for implementing mmap() support for vDPA device. This is because, an reference to the address space was held via mm_get() in vhost_dev_set_owner() and an reference to the file was held in mmap(). This means when process exits, the mm can not be released thus we can not release the file. This patch tries to use mmgrab() instead of mmget(), which allows the address space to be destroy in process exit without releasing the mm structure itself. This is sufficient for vDPA device which pin user pages and does not depend on the address space to work. Signed-off-by: Jason Wang <jasowang@redhat.com> Link: https://lore.kernel.org/r/20200529080303.15449-3-jasowang@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>
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