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

[David Laight <David.Laight@xxxxxxxxxx>]

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...

	David

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