linux-kernel - Re: [PATCH] x86/mm: Define PTRS_PER

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Message-ID: <Z8skF4rtRzaDL2Ou@smile.fi.intel.com>
Date: Fri, 7 Mar 2025 18:51:35 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
	kernel test robot <lkp@...el.com>, x86@...nel.org
Subject: Re: [PATCH] x86/mm: Define PTRS_PER_PMD for assembly code too

On Thu, Mar 06, 2025 at 11:00:16PM +0100, Ingo Molnar wrote:
> * Ingo Molnar <mingo@...nel.org> wrote:
> 
> > Separating out the assembler-compatible defines from the types 
> > headers appears to be a bigger patch, since it's all mixed in with C 
> > syntax:
> > 
> > <=-----------------------------------===============================
> > typedef struct { pud_t pud; } pmd_t;
> > 
> > #define PMD_SHIFT       PUD_SHIFT
> > #define PTRS_PER_PMD    1
> > #define PMD_SIZE        (1UL << PMD_SHIFT)
> > #define PMD_MASK        (~(PMD_SIZE-1))
> > 
> > /*
> >  * The "pud_xxx()" functions here are trivial for a folded two-level
> >  * setup: the pmd is never bad, and a pmd always exists (as it's folded
> >  * into the pud entry)
> >  */
> > static inline int pud_none(pud_t pud)           { return 0; }
> > static inline int pud_bad(pud_t pud)            { return 0; }
> > static inline int pud_present(pud_t pud)        { return 1; }
> > ================================================================>
> > 
> > In any case I've removed the commit for the time being until this all 
> > is cleared up.
> 
> So there's a simple solution: define it on i386 too, via the patch 
> below. It appears the double-definition doesn't create any warnings, on 
> GCC at least.

Fine by me as long as it gets fixed. Currently it prevents the WERROR=y
to be used along with `make W=1` for x86_32 by both compilers.

> But if it's an issue, we could do something like this in 
> <asm-generic/pgtable-nopmd.h>:
> 
>  #if defined(PTRS_PER_PMD) && (PTRS_PER_PMD != 1)
>  # error "mm: Wait a minute, that's a super confusing pagetable setup ..."
>  #endif
> 
> ?
> 
> Thanks,
> 
> 	Ingo
> 
> =========================>
> From: Ingo Molnar <mingo@...nel.org>
> Date: Thu, 6 Mar 2025 22:53:49 +0100
> Subject: [PATCH] x86/mm: Define PTRS_PER_PMD for assembly code too
> 
> Andy reported the following build warning from head_32.S:
> 
>   In file included from arch/x86/kernel/head_32.S:29:
>   arch/x86/include/asm/pgtable_32.h:59:5: error: "PTRS_PER_PMD" is not defined, evaluates to 0 [-Werror=undef]
>        59 | #if PTRS_PER_PMD > 1
> 
> The reason is that on 2-level i386 paging the folded in PMD's
> PTRS_PER_PMD constant is not defined in assembly headers,
> only in generic MM C headers.
> 
> Instead of trying to fish out the definition from the generic
> headers, just define it - it even has a comment for it already...
> 
> Reported-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> ---
>  arch/x86/include/asm/pgtable-2level_types.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/pgtable-2level_types.h b/arch/x86/include/asm/pgtable-2level_types.h
> index 7f6ccff0ba72..4a12c276b181 100644
> --- a/arch/x86/include/asm/pgtable-2level_types.h
> +++ b/arch/x86/include/asm/pgtable-2level_types.h
> @@ -23,17 +23,17 @@ typedef union {
>  #define ARCH_PAGE_TABLE_SYNC_MASK	PGTBL_PMD_MODIFIED
>  
>  /*
> - * traditional i386 two-level paging structure:
> + * Traditional i386 two-level paging structure:
>   */
>  
>  #define PGDIR_SHIFT	22
>  #define PTRS_PER_PGD	1024
>  
> -
>  /*
> - * the i386 is two-level, so we don't really have any
> - * PMD directory physically.
> + * The i386 is two-level, so we don't really have any
> + * PMD directory physically:
>   */
> +#define PTRS_PER_PMD	1

Should I give a try?

Okay, just

Tested-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

for x86_32 with Clang 19.1.7 and GCC 14.2.0.

-- 
With Best Regards,
Andy Shevchenko