linux-kernel - Re: [PATCH 6/7] mm: convert generic code to 5-level paging

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Date:   Thu, 9 Mar 2017 14:47:16 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/7] mm: convert generic code to 5-level paging

On Thu, Mar 09, 2017 at 10:54:15AM +0100, Michal Hocko wrote:
> On Wed 08-03-17 18:21:30, Kirill A. Shutemov wrote:
> > On Wed, Mar 08, 2017 at 02:57:35PM +0100, Michal Hocko wrote:
> > > On Mon 06-03-17 23:45:13, Kirill A. Shutemov wrote:
> > > > Convert all non-architecture-specific code to 5-level paging.
> > > > 
> > > > It's mostly mechanical adding handling one more page table level in
> > > > places where we deal with pud_t.
> > > > 
> > > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > > 
> > > OK, I haven't spotted anything major. I am just scratching my head about
> > > the __ARCH_HAS_5LEVEL_HACK leak into kasan_init.c (see below). Why do we
> > > need it?  It looks more than ugly but I am not familiar with kasan so
> > > maybe this is really necessary.
> > 
> > Yeah ugly.
> > 
> > kasan_zero_p4d is only defined if we have real page table level. It's okay
> > if the page table level is folded properly -- using pgtable-nop4d.h -- in
> > this case pgd_populate() is nop and we don't reference kasan_zero_p4d.
> > 
> > With 5level-fixup.h, pgd_populate() is not nop, so we would reference
> > kasan_zero_p4d and build breaks. We don't need this as p4d_populate()
> > would do what we really need in this case.
> > 
> > We can drop the hack once all architectures that support kasan would be
> > converted to pgtable-nop4d.h -- amd64 and x86 at the moment.
> 
> But those architectures even do not enable kasan
> $ git grep "select *HAVE_ARCH_KASAN"
> arch/arm64/Kconfig:     select HAVE_ARCH_KASAN if SPARSEMEM_VMEMMAP && !(ARM64_16K_PAGES && ARM64_VA_BITS_48)
> arch/x86/Kconfig:       select HAVE_ARCH_KASAN                  if X86_64 && SPARSEMEM_VMEMMAP
> 
> both arm64 and x86 (64b) do compile fine without the ifdef... So I guess
> we should be fine without it.

Have you build the image to the final linking? lm_alias() hides the error
until later.

x86-64 allmodconfig without the #ifndef:

  MODPOST vmlinux.o
mm/built-in.o: In function `kasan_populate_zero_shadow':
(.init.text+0xb72b): undefined reference to `kasan_zero_p4d'
Makefile:983: recipe for target 'vmlinux' failed
make: *** [vmlinux] Error 1

-- 
 Kirill A. Shutemov