Re: kmap_atomic and preemption

[Russell King - ARM Linux]

On Wed, May 04, 2016 at 03:47:29PM +0200, Peter Zijlstra wrote:
> Traditionally kmap_atomic() disables preemption; and the reason is that
> the returned pointer must stay valid. This had a side effect in that it
> also disabled pagefaults.

A lowmem page should never change its page_address(), so that much is
safe.  I think the question is whether there is any driver code which
assumes that preemption is unconditionally disabled between a
kmap_atomic() has been called.

That wouldn't be an unreasonable assumption given the name of the
function, so I'd suggest caution with making kmap_atomic() have these
kinds of differing behaviours depending on whether we're asking to
kmap a high or lowmem page.

If we are going to allow this, I think it at least needs to be well
documented.

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