Re: [PATCH v3 0/7] binder: eliminate use of vmalloc space for binder buffers

[Christoph Hellwig]

On Fri, Feb 08, 2019 at 10:35:13AM -0800, Todd Kjos wrote:
> Binder buffers have always been mapped into kernel space
> via map_kernel_range_noflush() to allow the binder driver
> to modify the buffer before posting to userspace for
> processing.
> 
> In recent Android releases, the number of long-running
> binder processes has increased to the point that for
> 32-bit systems, there is a risk of running out of
> vmalloc space.
> 
> This patch set removes the persistent mapping of the
> binder buffers into kernel space. Instead, the binder
> driver creates temporary mappings with kmap() or
> kmap_atomic() to copy to or from the buffer only when
> necessary.

Is there any good reason to actually map the user memory to kernel
space instead of just using copy_{to,from}_user?