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

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Date:   Mon, 23 Oct 2017 15:21:59 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Ingo Molnar <mingo@...nel.org>
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

On Mon, Oct 23, 2017 at 01:56:58PM +0200, Ingo Molnar wrote:
> 
> * 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 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