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Date:   Fri, 1 Mar 2019 15:28:44 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Mike Rapoport <rppt@...ux.ibm.com>
Cc:     Steven Price <steven.price@....com>,
        Mark Rutland <Mark.Rutland@....com>, x86@...nel.org,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Jérôme Glisse <jglisse@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        James Morse <james.morse@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arm-kernel@...ts.infradead.org,
        "Liang, Kan" <kan.liang@...ux.intel.com>
Subject: Re: [PATCH v2 03/13] mm: Add generic p?d_large() macros

On Fri, Mar 01, 2019 at 01:49:53PM +0200, Mike Rapoport wrote:
> Hi Kirill,
> 
> On Thu, Feb 21, 2019 at 05:57:06PM +0300, Kirill A. Shutemov wrote:
> > On Thu, Feb 21, 2019 at 02:46:18PM +0000, Steven Price wrote:
> > > On 21/02/2019 14:28, Kirill A. Shutemov wrote:
> > > > On Thu, Feb 21, 2019 at 11:34:52AM +0000, Steven Price wrote:
> > > >> From: James Morse <james.morse@....com>
> > > >>
> > > >> Exposing the pud/pgd levels of the page tables to walk_page_range() means
> > > >> we may come across the exotic large mappings that come with large areas
> > > >> of contiguous memory (such as the kernel's linear map).
> > > >>
> > > >> For architectures that don't provide p?d_large() macros, provided a
> > > >> does nothing default.
> > > > 
> > > > Nak, sorry.
> > > > 
> > > > Power will get broken by the patch. It has pmd_large() inline function,
> > > > that will be overwritten by the define from this patch.
> > > > 
> > > > I believe it requires more ground work on arch side in general.
> > > > All architectures that has huge page support has to provide these helpers
> > > > (and matching defines) before you can use it in a generic code.
> > > 
> > > Sorry about that, I had compile tested on power, but obviously not the
> > > right config to actually see the breakage.
> > 
> > I don't think you'll catch it at compile-time. It would silently override
> > the helper with always-false.
> 
> Can you explain why the compiler would override the helper define in, e.g.
> arch/powerpc/include/asm/pgtable.h with the generic (0)?

This one will not be overrided, but the other one will. See

arch/powerpc/include/asm/book3s/64/pgtable.h

-- 
 Kirill A. Shutemov

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