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

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Message-ID: <20170308152129.sknp75d5usdu4vne@black.fi.intel.com>
Date:   Wed, 8 Mar 2017 18:21:30 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     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 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.

Makes sense?

-- 
 Kirill A. Shutemov