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Date: Mon, 23 Oct 2017 15:21:59 +0300
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Ingo Molnar <mingo@kernel.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>, x86@kernel.org,
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        Cyrill Gorcunov <gorcunov@openvz.org>, Borislav Petkov <bp@suse.de>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/6] Boot-time switching between 4- and 5-level paging
 for 4.15, Part 1
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On Mon, Oct 23, 2017 at 01:56:58PM +0200, Ingo Molnar wrote:
> 
> * Kirill A. Shutemov <kirill@shutemov.name> wrote:
> 
> > > Or, could we keep MAX_PHYSMEM_BITS constant, and introduce a _different_ constant 
> > > that is dynamic, and which could be used in the cases where the 5-level paging 
> > > config causes too much memory footprint in the common 4-level paging case?
> > 
> > This is more labor intensive case with unclear benefit.
> > 
> > Dynamic MAX_PHYSMEM_BITS doesn't cause any issue in wast majority of
> > cases.
> 
> Almost nothing uses it - and even in those few cases it caused problems.

It's used in many places indirectly. See MAXMEM.

> Making a variable that 'looks' like a constant macro dynamic in a rare Kconfig 
> scenario is asking for trouble.

We expect boot-time page mode switching to be enabled in kernel of next
generation enterprise distros. It shoudn't be that rare.

-- 
 Kirill A. Shutemov