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

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Ingo Molnar <mingo@kernel.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Ingo Molnar <mingo@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	x86@kernel.org, Thomas Gleixner <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andy Lutomirski <luto@amacapital.net>,
	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
Date: Tue, 24 Oct 2017 14:38:19 +0300	[thread overview]
Message-ID: <20171024113819.pli7ifesp2u2rexi@node.shutemov.name> (raw)
In-Reply-To: <20171024094039.4lonzocjt5kras7m@gmail.com>

On Tue, Oct 24, 2017 at 11:40:40AM +0200, Ingo Molnar wrote:
> 
> * 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.

Would we need to rename every uppercase macros that would depend on
max_physmem_bits()? Like MAXMEM.

> > 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.

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

If you still insist, I'll rework code as you describe, but I disagree
that's the best way to go.

We also need to make other upper case macros dynamic, like PGDIR_SHIFT or
PTRS_PER_P4D. Reworking them in the same would be *far* more complex as
they (and their derivatives) used heavily in generic code.

To me it's a lot of code for a small to none benefit.

P.S. Could you please take a look on x86/boot/compressed/64 changes?

-- 
 Kirill A. Shutemov