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:   Fri, 20 Oct 2017 19:23:49 +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 Fri, Oct 20, 2017 at 05:23:46PM +0200, Ingo Molnar wrote:
> 
> * Kirill A. Shutemov <kirill.shutemov@...ux.intel.com> wrote:
> 
> > On Fri, Oct 20, 2017 at 08:18:53AM +0000, Ingo Molnar wrote:
> > > 
> > > * Kirill A. Shutemov <kirill@...temov.name> wrote:
> > > 
> > > > On Tue, Oct 03, 2017 at 11:27:54AM +0300, Kirill A. Shutemov wrote:
> > > > > On Fri, Sep 29, 2017 at 05:08:15PM +0300, Kirill A. Shutemov wrote:
> > > > > > The first bunch of patches that prepare kernel to boot-time switching
> > > > > > between paging modes.
> > > > > > 
> > > > > > Please review and consider applying.
> > > > > 
> > > > > Ping?
> > > > 
> > > > Ingo, is there anything I can do to get review easier for you?
> > > 
> > > Yeah, what is the conclusion on the sub-discussion of patch #2:
> > > 
> > >   [PATCH 2/6] mm/zsmalloc: Prepare to variable MAX_PHYSMEM_BITS
> > > 
> > > ... do we want to skip it entirely and use the other 5 patches?
> > 
> > Yes, please. MAX_PHYSMEM_BITS not variable yet in this part of the series.
> > 
> > And I will post some version the patch in the next part, if it will be
> > required.
> 
> Could we add TRULY_MAX_PHYSMEM_BITS (with a better name), to be used in places 
> where memory footprint is not a big concern?

That's what I did in the patch. See MAX_POSSIBLE_PHYSMEM_BITS.
Not sure how good the name is.

> 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.

-- 
 Kirill A. Shutemov