On Mon, Dec 13, 2021 at 08:30:33AM +0000, David Laight wrote:

From: Matthew Wilcox

quoted

Sent: 12 December 2021 11:48

On Sat, Dec 11, 2021 at 05:53:46PM +0000, David Laight wrote:

quoted

From: Tiezhu Yang

quoted

Sent: 11 December 2021 03:33

v2:
  -- add copy_to_user_or_kernel() in lib/usercopy.c
  -- define userbuf as bool type

Instead of having a flag to indicate whether the buffer is user or kernel,
would it be better to have two separate buffer pointers.
One for a user space buffer, the other for a kernel space buffer.
Exactly one of the buffers should always be NULL.

No.  You should be using an iov_iter instead.  See
https://lore.kernel.org/all/Ya4bdB0UBJCZhUSo@casper.infradead.org/ (local)
for a start on this.

iov_iter gets horribly expensive...

Oh, right.  Reading the kcore is a high-performance path, my mistake.