Re: [PATCH v2 0/2] kdump: simplify code

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

On Mon, Dec 13, 2021 at 08:30:33AM +0000, David Laight wrote:
> From: Matthew Wilcox
> > Sent: 12 December 2021 11:48
> > 
> > On Sat, Dec 11, 2021 at 05:53:46PM +0000, David Laight wrote:
> > > From: Tiezhu Yang
> > > > 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@xxxxxxxxxxxxxxxxxxxx/
> > for a start on this.
> 
> iov_iter gets horribly expensive...

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