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: <20171024124741.ux74rtbu2vqaf6zt@gmail.com>
Date:   Tue, 24 Oct 2017 14:47:41 +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:

> On Tue, Oct 24, 2017 at 11:40:40AM +0200, Ingo Molnar wrote:
> > 
> > * Kirill A. Shutemov <kirill@...temov.name> wrote:
> > 
> > > On Mon, Oct 23, 2017 at 02:40:14PM +0200, Ingo Molnar wrote:
> > > > 
> > > > * Kirill A. Shutemov <kirill@...temov.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.
> 
> Would we need to rename every uppercase macros that would depend on
> max_physmem_bits()? Like MAXMEM.

MAXMEM isn't used in too many places either - what's the total impact of it?

> > > 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?
> 
> I don't think it's the worst case.
> 
> For instance, virt_addr_valid() depends indirectly on it:
> 
>   virt_addr_valid()
>     __virt_addr_valid()
>       phys_addr_valid()
>         boot_cpu_data.x86_phys_bits (initialized with MAX_PHYSMEM_BITS)
> 
> virt_addr_valid() is used in things like implementation /dev/kmem.
> 
> To me it's far more risky than occasional build breakage for
> CONFIG_X86_5LEVEL=y.

So why do we have two variables here, one boot_cpu_data.x86_phys_bits and the 
other MAX_PHYSMEM_BITS - both set once during boot?

I'm trying to find a clean solution for this all - hiding a boot time dependency 
into a constant-looking value doesn't feel clean.

Thanks,

	Ingo