[PATCH 02/16] drm/amdkfd: Don't dereference kfd_process.mm

[Christian König]

Am 21.10.2017 um 02:23 schrieb Felix Kuehling:
> The kfd_process doesn't own a reference to the mm_struct, so it can
> disappear without warning even while the kfd_process still exists.
> In fact, the delayed kfd_process teardown is triggered by an MMU
> notifier when the mm_struct is destroyed. Permanently holding a
> reference to the mm_struct would prevent this from happening.
>
> Therefore, avoid dereferencing the kfd_process.mm pointer and make
> it opaque. Use get_task_mm to get a temporary reference to the mm
> when it's needed.

Actually that patch is unnecessary.

Process tear down (and calling the MMU release callback) is triggered 
when mm_struct->mm_users reaches zero.

The mm_struct is freed up when mm_struct->mm_count becomes zero.

So what you can do is grab a reference to the mm_struct with mmgrab() 
and still be notified by process tear down.

Regards,
Christian.

>
> Signed-off-by: Felix Kuehling <Felix.Kuehling at amd.com>
> ---
>   drivers/gpu/drm/amd/amdkfd/kfd_events.c  | 19 +++++++++++++++----
>   drivers/gpu/drm/amd/amdkfd/kfd_priv.h    |  7 ++++++-
>   drivers/gpu/drm/amd/amdkfd/kfd_process.c |  6 +++++-
>   3 files changed, 26 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_events.c b/drivers/gpu/drm/amd/amdkfd/kfd_events.c
> index 944abfa..61ce547 100644
> --- a/drivers/gpu/drm/amd/amdkfd/kfd_events.c
> +++ b/drivers/gpu/drm/amd/amdkfd/kfd_events.c
> @@ -24,8 +24,8 @@
>   #include <linux/slab.h>
>   #include <linux/types.h>
>   #include <linux/sched/signal.h>
> +#include <linux/sched/mm.h>
>   #include <linux/uaccess.h>
> -#include <linux/mm.h>
>   #include <linux/mman.h>
>   #include <linux/memory.h>
>   #include "kfd_priv.h"
> @@ -904,14 +904,24 @@ void kfd_signal_iommu_event(struct kfd_dev *dev, unsigned int pasid,
>   	 * running so the lookup function returns a locked process.
>   	 */
>   	struct kfd_process *p = kfd_lookup_process_by_pasid(pasid);
> +	struct mm_struct *mm;
>   
>   	if (!p)
>   		return; /* Presumably process exited. */
>   
> +	/* Take a safe reference to the mm_struct, which may otherwise
> +	 * disappear even while the kfd_process is still referenced.
> +	 */
> +	mm = get_task_mm(p->lead_thread);
> +	if (!mm) {
> +		mutex_unlock(&p->mutex);
> +		return; /* Process is exiting */
> +	}
> +
>   	memset(&memory_exception_data, 0, sizeof(memory_exception_data));
>   
> -	down_read(&p->mm->mmap_sem);
> -	vma = find_vma(p->mm, address);
> +	down_read(&mm->mmap_sem);
> +	vma = find_vma(mm, address);
>   
>   	memory_exception_data.gpu_id = dev->id;
>   	memory_exception_data.va = address;
> @@ -937,7 +947,8 @@ void kfd_signal_iommu_event(struct kfd_dev *dev, unsigned int pasid,
>   		}
>   	}
>   
> -	up_read(&p->mm->mmap_sem);
> +	up_read(&mm->mmap_sem);
> +	mmput(mm);
>   
>   	mutex_lock(&p->event_mutex);
>   
> diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_priv.h b/drivers/gpu/drm/amd/amdkfd/kfd_priv.h
> index 7d86ec9..1a483a7 100644
> --- a/drivers/gpu/drm/amd/amdkfd/kfd_priv.h
> +++ b/drivers/gpu/drm/amd/amdkfd/kfd_priv.h
> @@ -494,7 +494,12 @@ struct kfd_process {
>   	 */
>   	struct hlist_node kfd_processes;
>   
> -	struct mm_struct *mm;
> +	/*
> +	 * Opaque pointer to mm_struct. We don't hold a reference to
> +	 * it so it should never be dereferenced from here. This is
> +	 * only used for looking up processes by their mm.
> +	 */
> +	void *mm;
>   
>   	struct mutex mutex;
>   
> diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_process.c b/drivers/gpu/drm/amd/amdkfd/kfd_process.c
> index 3ccb3b5..21d27e5 100644
> --- a/drivers/gpu/drm/amd/amdkfd/kfd_process.c
> +++ b/drivers/gpu/drm/amd/amdkfd/kfd_process.c
> @@ -200,7 +200,11 @@ static void kfd_process_destroy_delayed(struct rcu_head *rcu)
>   	struct kfd_process *p;
>   
>   	p = container_of(rcu, struct kfd_process, rcu);
> -	WARN_ON(atomic_read(&p->mm->mm_count) <= 0);
> +	/*
> +	 * This cast should be safe here because we grabbed a
> +	 * reference to the mm in kfd_process_notifier_release
> +	 */
> +	WARN_ON(atomic_read(&((struct mm_struct *)p->mm)->mm_count) <= 0);
>   
>   	mmdrop(p->mm);
>