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    SubjectRe: [patch 00/13] preempt: Make preempt count unconditional
    On Wed, 16 Sep 2020 at 21:32, Linus Torvalds
    <torvalds@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.

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