Re: [PATCH] dma-direct: fix DMA_ATTR_NO_KERNEL_MAPPING

[Christoph Hellwig]

As others pointed out, DMA_ATTR_NO_KERNEL_MAPPING just means the
caller can't rely on a kernel mapping.  So the "fix" here is
wrong.  That being said for cases where we can easily remove a page
from the kernel mapping it would be nice to do to:

 a) improve security
 b) as a debug check to see that no one actually tries to access it

> +		/* remove kernel mapping for pages */
> +		set_memory_valid((unsigned long)phys_to_virt(dma_to_phys(dev, *dma_handle)),

Please avoid overly long lines.  Also this function only exists for arm64
also and others pointed out won't work for all cases.