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

From: Kirill A. Shutemov
Date: Tue Oct 29 2019 - 07:00:27 EST

On Tue, Oct 29, 2019 at 09:56:02AM +0100, Peter Zijlstra wrote:
> 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.

It's not clear if it is enough to avoid the issue. "under a highly specific
and detailed set of conditions" is not very specific set of conditions :P

Kirill A. Shutemov