linux-kernel - Re: [PATCH 0/6] Boot-time switching between 4- and 5-level paging for 4.15, Part 1

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Message-ID: <20171023115658.geccs22o2t733np3@gmail.com>
Date:   Mon, 23 Oct 2017 13:56:58 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Borislav Petkov <bp@...e.de>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] Boot-time switching between 4- and 5-level paging
 for 4.15, Part 1


* Kirill A. Shutemov <kirill@...temov.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 waste majority of
> cases.

Almost nothing uses it - and even in those few cases it caused problems.

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

Thanks,

	Ingo