[patch 00/13] preempt: Make preempt count unconditional

[Ard Biesheuvel]

On Wed, 16 Sep 2020 at 21:32, Linus Torvalds
<torvalds at linux-foundation.org> wrote:
>
> But something like a driver list walking thing should not be doing
> different things behind peoples back depending on whether they hold
> spinlocks or not. It should either just work regardless, or there
> should be a flag (or special interface) for the "you're being called
> in a crtitical region".
>
> Because dynamically changing behavior really is very confusing.
>

By the same reasoning, I don't think a generic crypto library should
be playing tricks with preemption en/disabling under the hood when
iterating over some data that is all directly accessible via the
linear map on the platforms that most people care about. And using
kmap_atomic() unconditionally achieves exactly that.

As I argued before, the fact that kmap_atomic() can be called from an
atomic context, and the fact that its implementation on HIGHMEM
platforms requires preemption to be disabled until the next kunmap()
are two different things, and I don't agree with your assertion that
the name kmap_atomic() implies the latter semantics. If we can avoid
disabling preemption on HIGHMEM, as Thomas suggests, we surely don't
need it on !HIGHMEM either, and given that kmap_atomic() is preferred
today anyway, we can just merge the two implementations. Are there any
existing debug features that could help us spot [ab]use of things like
raw per-CPU data within kmap_atomic regions?

Re your point about deprecating HIGHMEM: some work is underway on ARM
to implement a 3.75/3.75 GB kernel/user split on recent LPAE capable
hardware (which shouldn't suffer from the performance issues that
plagued the 4/4 split on i686), and so hopefully, there is a path
forward for ARM that does not rely on HIGHMEM as it does today.