Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user mappings

Peter Zijlstra
2019-10-28T13:55:44Z

On Mon, Oct 28, 2019 at 04:16:23PM +0300, Kirill A. Shutemov wrote:

> I think active use of this feature will lead to performance degradation of
> the system with time.
>
> Setting a single 4k page non-present in the direct mapping will require
> splitting 2M or 1G page we usually map direct mapping with. And it's one
> way road. We don't have any mechanism to map the memory with huge page
> again after the application has freed the page.

Right, we recently had a 'bug' where ftrace triggered something like
this and facebook ran into it as a performance regression. So yes, this
is a real concern.