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

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Message-ID: <20180312145222.GC22761@uranus.lan>
Date:   Mon, 12 Mar 2018 17:52:22 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Peter Zijlstra <peterz@...radead.org>,
        kirill.shutemov@...ux.intel.com, luto@...capital.net,
        keescook@...omium.org, willy@...radead.org,
        torvalds@...ux-foundation.org, tglx@...utronix.de, bp@...e.de,
        andy.shevchenko@...il.com, linux-kernel@...r.kernel.org,
        hpa@...or.com, ebiederm@...ssion.com, jgross@...e.com,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/mm] x86/boot/compressed/64: Describe the logic behind
 the LA57 check

On Mon, Mar 12, 2018 at 03:32:12PM +0100, Ingo Molnar wrote:
> 
> * Kirill A. Shutemov <kirill@...temov.name> 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?