Re: [PATCH 2/3] drm/i915/userptr: Hold mmref whilst calling get-user-pages

[Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>]

On 03/04/16 18:14, Chris Wilson wrote:
> Holding a reference to the containing task_struct is not sufficient to
> prevent the mm_struct from being reaped under memory pressure. If this
> happens whilst we are calling get_user_pages(), explosions errupt -
> sometimes an immediate GPF, sometimes page flag corruption. To prevent
> the target mm from being reaped as we are reading from it, acquire a
> reference before we begin.
>
> Testcase: igt/gem_shrink/*userptr
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> Cc: Michał Winiarski <michal.winiarski@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>   drivers/gpu/drm/i915/i915_gem_userptr.c | 29 +++++++++++++++++------------
>   1 file changed, 17 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
> index 92b39186b05a..960bb37f458f 100644
> --- a/drivers/gpu/drm/i915/i915_gem_userptr.c
> +++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
> @@ -547,19 +547,24 @@ __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
>   	if (pvec != NULL) {
>   		struct mm_struct *mm = obj->userptr.mm->mm;
>
> -		down_read(&mm->mmap_sem);
> -		while (pinned < npages) {
> -			ret = get_user_pages_remote(work->task, mm,
> -					obj->userptr.ptr + pinned * PAGE_SIZE,
> -					npages - pinned,
> -					!obj->userptr.read_only, 0,
> -					pvec + pinned, NULL);
> -			if (ret < 0)
> -				break;
> -
> -			pinned += ret;
> +		ret = -EFAULT;
> +		if (atomic_inc_not_zero(&mm->mm_users)) {
> +			down_read(&mm->mmap_sem);
> +			while (pinned < npages) {
> +				ret = get_user_pages_remote
> +					(work->task, mm,
> +					 obj->userptr.ptr + pinned * PAGE_SIZE,
> +					 npages - pinned,
> +					 !obj->userptr.read_only, 0,
> +					 pvec + pinned, NULL);
> +				if (ret < 0)
> +					break;
> +
> +				pinned += ret;
> +			}
> +			up_read(&mm->mmap_sem);
> +			mmput(mm);
>   		}
> -		up_read(&mm->mmap_sem);
>   	}
>
>   	mutex_lock(&dev->struct_mutex);

Strange, doesn't this mean that the atomic_inc(&current->mm->mm_count) 
is not doing what we thought it would?

Regards,

Tvrtko
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