title: cv2.imread() randomly make images become 'None'
body: <h2>Description</h2>
  <p>The system is Ubuntu 20.04 LTS. The framework is PyTorch.</p>
  <p>The problem is that cv2.imread() randomly makes 'None' images. It does not only return None, but also changed the images on the disk to be None, and cannot be read by cv2 next time.</p>
  <p>I was training some end-to-end CNN models (ResNet/DenseNet). Everything was running well if I used a 'small' dataset(about 200-500 images, randomly sampled from a 'big' dataset(about 5000 images)).</p>
  <p>However, if I used the 'big datasets', the error happened randomly, and the images on the disk will be destroyed permanently and became None type.</p>
  <h2>What I tried</h2>
  <p>I tried to write a function to avoid loading the destroyed images, which means that the training images would become fewer and fewer (about ten images per 100 epochs).</p>
  <p>Another operation that I tried is to set the permission of the image file as read-only, but got errors.</p>
  <p>I also tried different models but still got this problem randomly.</p>
  <h2>Another potential problem</h2>
  <p>I am not sure whether there is a relationship between fsck and the None image problem. every time I rebooted the system, I couldn't enter the ubuntu system but need to fsck manually.</p>
  <p>Is there anyone who knows the solution or faces the same problem before? Could you please help me solve it? Thank you!</p>