linux-kernel - Re: [PATCH v10 20/22] x86: mm: Convert dump_pagetables to use walk_page

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Message-Id: <20190807131658.08793793a97fa4310af4f495@linux-foundation.org>
Date:   Wed, 7 Aug 2019 13:16:58 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Steven Price <steven.price@....com>
Cc:     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>,
        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>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        "Liang, Kan" <kan.liang@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v10 20/22] x86: mm: Convert dump_pagetables to use
 walk_page_range

On Wed, 7 Aug 2019 13:58:21 +0100 Steven Price <steven.price@....com> wrote:

> > ./arch/x86/include/asm/pgtable_64_types.h:56:22: error: initializer element is not constant
> >  #define PTRS_PER_PGD 512
> >                       ^
> 
> This is very unhelpful of GCC - it's actually PTRS_PER_P4D which isn't
> constant!

Well.  You had every right to assume that an all-caps macro is a
compile-time constant.

We are innocent victims of Kirill's c65e774fb3f6af2 ("x86/mm: Make
PGDIR_SHIFT and PTRS_PER_P4D variable") which lazily converted these
macros into runtime-only, under some Kconfig settings.  It should have
changed those macros into static inlined lower-case functions.