It's incorrect to tell out if a task is kthread without checking
PF_KTHREAD at all.

What's also fixed, if no need to be in a seperate patch, is to
invoke kthread_use_mm() without checking the current mm first as
the kthread may hold a mm struct atm and it's not the right place
to switch mm.

Fixes: 8d46c0cca5f4 ("vfio: introduce vfio_dma_rw to read/write a range of 
IOVAs")
Cc: Yan Zhao <yan.y.z...@intel.com>
Cc: Markus Elfring <markus.elfr...@web.de>
Cc: Christoph Hellwig <h...@lst.de>
Signed-off-by: Hillf Danton <hdan...@sina.com>
---

--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -2798,7 +2798,8 @@ static int vfio_iommu_type1_dma_rw_chunk
        struct mm_struct *mm;
        unsigned long vaddr;
        struct vfio_dma *dma;
-       bool kthread = current->mm == NULL;
+       bool kthread = current->flags & PF_KTHREAD;
+       bool use_mm = current->mm == NULL;
        size_t offset;
 
        *copied = 0;
@@ -2812,11 +2813,10 @@ static int vfio_iommu_type1_dma_rw_chunk
                return -EPERM;
 
        mm = get_task_mm(dma->task);
-
        if (!mm)
                return -EPERM;
 
-       if (kthread)
+       if (kthread && use_mm)
                kthread_use_mm(mm);
        else if (current->mm != mm)
                goto out;
@@ -2843,7 +2843,7 @@ static int vfio_iommu_type1_dma_rw_chunk
        } else
                *copied = copy_from_user(data, (void __user *)vaddr,
                                           count) ? 0 : count;
-       if (kthread)
+       if (kthread && use_mm)
                kthread_unuse_mm(mm);
 out:
        mmput(mm);
--

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