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

[Peter Zijlstra]

On Tue, Oct 29, 2019 at 09:43:18AM +0300, Kirill A. Shutemov wrote:
> But some CPUs don't like to have two TLB entries for the same memory with
> different sizes at the same time. See for instance AMD erratum 383.
> 
> Getting it right would require making the range not present, flush TLB and
> only then install huge page. That's what we do for userspace.
> 
> It will not fly for the direct mapping. There is no reasonable way to
> exclude other CPU from accessing the range while it's not present (call
> stop_machine()? :P). Moreover, the range may contain the code that doing
> the collapse or data required for it...
> 
> BTW, looks like current __split_large_page() in pageattr.c is susceptible
> to the errata. Maybe we can get away with the easy way...

As you write above, there is just no way we can have a (temporary) hole
in the direct map.

We are careful about that other errata, and make sure both translations
are identical wrt everything else.