improve use_mm / unuse_mm v2

[Matthew Wilcox]

On Thu, Apr 16, 2020 at 07:31:55AM +0200, Christoph Hellwig wrote:
> this series improves the use_mm / unuse_mm interface by better
> documenting the assumptions, and my taking the set_fs manipulations
> spread over the callers into the core API.

I appreciate all the work you're doing here.

Do you have plans to introduce a better-named API than set_fs() / get_fs()?

Also, having set_fs() return the previous value of 'fs' would simplify
a lot of the callers.