On Wed, 16 Sep 2020 at 21:32, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> 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.
