linux-kernel - Re: [PATCHv2 03/29] asm-generic: introduce __ARCH_USE_5LEVEL

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Date:   Fri, 27 Jan 2017 14:24:58 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     "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>
Cc:     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: [PATCHv2 03/29] asm-generic: introduce __ARCH_USE_5LEVEL_HACK

On 12/27/2016 02:53 AM, Kirill A. Shutemov wrote:
> We are going to introduce <asm-generic/pgtable-nop4d.h> to provide
> abstraction for properly (in opposite to 5level-fixup.h hack) folded
> p4d level. The new header will be included from pgtable-nopud.h.
>
> If an architecture uses <asm-generic/nop*d.h>, we cannot use
> 5level-fixup.h directly to quickly convert the architecture to 5-level
> paging as it would conflict with pgtable-nop4d.h.
>
> With this patch an architecture can define __ARCH_USE_5LEVEL_HACK before
> inclusion <asm-genenric/nop*d.h> to 5level-fixup.h.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  include/asm-generic/pgtable-nop4d-hack.h | 62 ++++++++++++++++++++++++++++++++

At risk of bikeshedding and coming from somebody not familiar with this code... 
IMHO it would be somewhat more intuitive and consistent to name the file 
"pgtable-nopud-hack.h" as it's about the pud stuff, not p4d stuff, and acts as 
an alternative implementation to pgtable-nopud.h, not pgtable-nop4d.h

Thanks,
Vlastimil

>  include/asm-generic/pgtable-nopud.h      |  5 +++
>  2 files changed, 67 insertions(+)
>  create mode 100644 include/asm-generic/pgtable-nop4d-hack.h
>
> diff --git a/include/asm-generic/pgtable-nop4d-hack.h b/include/asm-generic/pgtable-nop4d-hack.h
> new file mode 100644
> index 000000000000..752fb7511750
> --- /dev/null
> +++ b/include/asm-generic/pgtable-nop4d-hack.h
> @@ -0,0 +1,62 @@
> +#ifndef _PGTABLE_NOP4D_HACK_H
> +#define _PGTABLE_NOP4D_HACK_H
> +
> +#ifndef __ASSEMBLY__
> +#include <asm-generic/5level-fixup.h>
> +
> +#define __PAGETABLE_PUD_FOLDED
> +
> +/*
> + * Having the pud type consist of a pgd gets the size right, and allows
> + * us to conceptually access the pgd entry that this pud is folded into
> + * without casting.
> + */
> +typedef struct { pgd_t pgd; } pud_t;
> +
> +#define PUD_SHIFT	PGDIR_SHIFT
> +#define PTRS_PER_PUD	1
> +#define PUD_SIZE	(1UL << PUD_SHIFT)
> +#define PUD_MASK	(~(PUD_SIZE-1))
> +
> +/*
> + * The "pgd_xxx()" functions here are trivial for a folded two-level
> + * setup: the pud is never bad, and a pud always exists (as it's folded
> + * into the pgd entry)
> + */
> +static inline int pgd_none(pgd_t pgd)		{ return 0; }
> +static inline int pgd_bad(pgd_t pgd)		{ return 0; }
> +static inline int pgd_present(pgd_t pgd)	{ return 1; }
> +static inline void pgd_clear(pgd_t *pgd)	{ }
> +#define pud_ERROR(pud)				(pgd_ERROR((pud).pgd))
> +
> +#define pgd_populate(mm, pgd, pud)		do { } while (0)
> +/*
> + * (puds are folded into pgds so this doesn't get actually called,
> + * but the define is needed for a generic inline function.)
> + */
> +#define set_pgd(pgdptr, pgdval)	set_pud((pud_t *)(pgdptr), (pud_t) { pgdval })
> +
> +static inline pud_t *pud_offset(pgd_t *pgd, unsigned long address)
> +{
> +	return (pud_t *)pgd;
> +}
> +
> +#define pud_val(x)				(pgd_val((x).pgd))
> +#define __pud(x)				((pud_t) { __pgd(x) })
> +
> +#define pgd_page(pgd)				(pud_page((pud_t){ pgd }))
> +#define pgd_page_vaddr(pgd)			(pud_page_vaddr((pud_t){ pgd }))
> +
> +/*
> + * allocating and freeing a pud is trivial: the 1-entry pud is
> + * inside the pgd, so has no extra memory associated with it.
> + */
> +#define pud_alloc_one(mm, address)		NULL
> +#define pud_free(mm, x)				do { } while (0)
> +#define __pud_free_tlb(tlb, x, a)		do { } while (0)
> +
> +#undef  pud_addr_end
> +#define pud_addr_end(addr, end)			(end)
> +
> +#endif /* __ASSEMBLY__ */
> +#endif /* _PGTABLE_NOP4D_HACK_H */
> diff --git a/include/asm-generic/pgtable-nopud.h b/include/asm-generic/pgtable-nopud.h
> index 810431d8351b..5e49430a30a4 100644
> --- a/include/asm-generic/pgtable-nopud.h
> +++ b/include/asm-generic/pgtable-nopud.h
> @@ -3,6 +3,10 @@
>
>  #ifndef __ASSEMBLY__
>
> +#ifdef __ARCH_USE_5LEVEL_HACK
> +#include <asm-generic/pgtable-nop4d-hack.h>
> +#else
> +
>  #define __PAGETABLE_PUD_FOLDED
>
>  /*
> @@ -58,4 +62,5 @@ static inline pud_t * pud_offset(pgd_t * pgd, unsigned long address)
>  #define pud_addr_end(addr, end)			(end)
>
>  #endif /* __ASSEMBLY__ */
> +#endif /* !__ARCH_USE_5LEVEL_HACK */
>  #endif /* _PGTABLE_NOPUD_H */
>