Re: [tip:x86/mm] x86/boot/compressed/64: Describe the logic behind the LA57 check

[Cyrill Gorcunov]

On Mon, Mar 12, 2018 at 03:32:12PM +0100, Ingo Molnar wrote:
> 
> * Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
> 
> > > We can of course bike shed / benchmark this once my desktop refresh
> > > sports this feature, but ISTR this being one of the very first things
> > > Ingo mentioned when we started this whole 5L thing.
> > 
> > I would rather not fix the problem that may not actually exist. :)
> 
> That 5 level pagetables involve more overhead is a realy problem.
> 
> By default we should only enable 5-level paging if memory mappings exist in
> the memory map that require the extended physical memory space.

Does it mean that if a machine supports 5lvl but has phisycal memory
installed fitting the 4lvl space, and has memory hotplug supported,
adding more memory won't have effect until next reboot?