On 10/28/2016 7:48 AM, Alexey Kardashevskiy wrote: > On 27/10/16 23:31, Kirti Wankhede wrote: >> >> >> On 10/27/2016 12:50 PM, Alexey Kardashevskiy wrote: >>> On 18/10/16 08:22, Kirti Wankhede wrote: >>>> VFIO IOMMU drivers are designed for the devices which are IOMMU capable. >>>> Mediated device only uses IOMMU APIs, the underlying hardware can be >>>> managed by an IOMMU domain. >>>> >>>> Aim of this change is: >>>> - To use most of the code of TYPE1 IOMMU driver for mediated devices >>>> - To support direct assigned device and mediated device in single module >>>> >>>> Added two new callback functions to struct vfio_iommu_driver_ops. Backend >>>> IOMMU module that supports pining and unpinning pages for mdev devices >>>> should provide these functions. >>>> Added APIs for pining and unpining pages to VFIO module. These calls back >>>> into backend iommu module to actually pin and unpin pages. >>>> >>>> This change adds pin and unpin support for mediated device to TYPE1 IOMMU >>>> backend module. More details: >>>> - When iommu_group of mediated devices is attached, task structure is >>>> cached which is used later to pin pages and page accounting. >>> >>> >>> For SPAPR TCE IOMMU driver, I ended up caching mm_struct with >>> atomic_inc(&container->mm->mm_count) (patches are on the way) instead of >>> using @current or task as the process might be gone while VFIO container is >>> still alive and @mm might be needed to do proper cleanup; this might not be >>> an issue with this patchset now but still you seem to only use @mm from >>> task_struct. >>> >> >> Consider the example of QEMU process which creates VFIO container, QEMU >> in its teardown path would release the container. How could container be >> alive when process is gone? > > do_exit() in kernel/exit.c calls exit_mm() (which sets NULL to tsk->mm) > first, and then releases open files by calling exit_files(). So > container's release() does not have current->mm. >
Incrementing usage count (get_task_struct()) while saving task structure and decementing it (put_task_struct()) from release() should work here. Updating the patch. Thanks, Kirti
