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Date: Tue, 24 Oct 2017 11:40:40 +0200
From: Ingo Molnar <mingo@kernel.org>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
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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* Kirill A. Shutemov <kirill@shutemov.name> wrote:

> On Mon, Oct 23, 2017 at 02:40:14PM +0200, Ingo Molnar wrote:
> > 
> > * Kirill A. Shutemov <kirill@shutemov.name> wrote:
> > 
> > > > 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.
> > 
> > My point remains even with not-so-rare Kconfig dependency.
> 
> I don't follow how introducing new variable that depends on Kconfig option
> would help with the situation.

A new, properly named variable or function (max_physmem_bits or 
max_physmem_bits()) that is not all uppercase would make it abundantly clear that 
it is not a constant but a runtime value.

> We would end up with inverse situation: people would use MAX_PHYSMEM_BITS
> where the new variable need to be used and we will in the same situation.

It should result in sub-optimal resource allocations worst-case, right?

We could also rename it to MAX_POSSIBLE_PHYSMEM_BITS to make it clear that the 
real number of bits can be lower.

Thanks,

	Ingo