Date:	Thu, 22 Mar 2012 02:02:55 +1100
From:	Stephen Rothwell <sfr@...b.auug.org.au>
To:	Mark Salter <msalter@...hat.com>
Cc:	aarcange@...hat.com, linux-kernel <linux-kernel@...r.kernel.org>,
	linux-next <linux-next@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: build failure in linux-next

[cc: akpm]

On Wed, 21 Mar 2012 10:36:14 -0400 Mark Salter <msalter@...hat.com> wrote:
>
> I'm seeing a build failure in linux-next:
> 
>   CC      init/main.o
> In file included from /es/linux/linux-next/arch/c6x/include/asm/pgtable.h:76:0,
>                  from /es/linux/linux-next/include/linux/mm.h:44,
>                  from /es/linux/linux-next/include/linux/ring_buffer.h:5,
>                  from /es/linux/linux-next/include/linux/ftrace_event.h:4,
>                  from /es/linux/linux-next/include/trace/syscall.h:6,
>                  from /es/linux/linux-next/include/linux/syscalls.h:78,
>                  from /es/linux/linux-next/init/main.c:16:
> /es/linux/linux-next/include/asm-generic/pgtable.h: In function 'pmd_none_or_trans_huge_or_clear_bad':
> /es/linux/linux-next/include/asm-generic/pgtable.h:476:4: error: implicit declaration of function 'pmd_clear_bad' [-Werror=implicit-function-declaration]
> 
> 
> This patch added some functions to asm-generic/pgtable.h which should
> have been placed in the CONFIG_MMU conditional block:
> 
>   Author: Andrea Arcangeli <aarcange@...hat.com>
>   Date:   Wed Mar 21 10:48:00 2012 +1100
> 
>       mm: thp: fix pmd_bad() triggering in code paths holding mmap_sem read mode
> 
> 
> The following patch fixes the build problem for me:
> 
> diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
> index 202c010..8ba3ba5 100644
> --- a/include/asm-generic/pgtable.h
> +++ b/include/asm-generic/pgtable.h
> @@ -342,6 +342,64 @@ static inline void ptep_modify_prot_commit(struct mm_struct *mm,
>  	__ptep_modify_prot_commit(mm, addr, ptep, pte);
>  }
>  #endif /* __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION */
> +
> +/*
> + * This function is meant to be used by sites walking pagetables with
> + * the mmap_sem hold in read mode to protect against MADV_DONTNEED and
> + * transhuge page faults. MADV_DONTNEED can convert a transhuge pmd
> + * into a null pmd and the transhuge page fault can convert a null pmd
> + * into an hugepmd or into a regular pmd (if the hugepage allocation
> + * fails). While holding the mmap_sem in read mode the pmd becomes
> + * stable and stops changing under us only if it's not null and not a
> + * transhuge pmd. When those races occurs and this function makes a
> + * difference vs the standard pmd_none_or_clear_bad, the result is
> + * undefined so behaving like if the pmd was none is safe (because it
> + * can return none anyway). The compiler level barrier() is critically
> + * important to compute the two checks atomically on the same pmdval.
> + */
> +static inline int pmd_none_or_trans_huge_or_clear_bad(pmd_t *pmd)
> +{
> +	/* depend on compiler for an atomic pmd read */
> +	pmd_t pmdval = *pmd;
> +	/*
> +	 * The barrier will stabilize the pmdval in a register or on
> +	 * the stack so that it will stop changing under the code.
> +	 */
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	barrier();
> +#endif
> +	if (pmd_none(pmdval))
> +		return 1;
> +	if (unlikely(pmd_bad(pmdval))) {
> +		if (!pmd_trans_huge(pmdval))
> +			pmd_clear_bad(pmd);
> +		return 1;
> +	}
> +	return 0;
> +}
> +
> +/*
> + * This is a noop if Transparent Hugepage Support is not built into
> + * the kernel. Otherwise it is equivalent to
> + * pmd_none_or_trans_huge_or_clear_bad(), and shall only be called in
> + * places that already verified the pmd is not none and they want to
> + * walk ptes while holding the mmap sem in read mode (write mode don't
> + * need this). If THP is not enabled, the pmd can't go away under the
> + * code even if MADV_DONTNEED runs, but if THP is enabled we need to
> + * run a pmd_trans_unstable before walking the ptes after
> + * split_huge_page_pmd returns (because it may have run when the pmd
> + * become null, but then a page fault can map in a THP and not a
> + * regular page).
> + */
> +static inline int pmd_trans_unstable(pmd_t *pmd)
> +{
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	return pmd_none_or_trans_huge_or_clear_bad(pmd);
> +#else
> +	return 0;
> +#endif
> +}
> +
>  #endif /* CONFIG_MMU */
>  
>  /*
> @@ -444,63 +502,6 @@ static inline int pmd_write(pmd_t pmd)
>  #endif /* __HAVE_ARCH_PMD_WRITE */
>  #endif
>  
> -/*
> - * This function is meant to be used by sites walking pagetables with
> - * the mmap_sem hold in read mode to protect against MADV_DONTNEED and
> - * transhuge page faults. MADV_DONTNEED can convert a transhuge pmd
> - * into a null pmd and the transhuge page fault can convert a null pmd
> - * into an hugepmd or into a regular pmd (if the hugepage allocation
> - * fails). While holding the mmap_sem in read mode the pmd becomes
> - * stable and stops changing under us only if it's not null and not a
> - * transhuge pmd. When those races occurs and this function makes a
> - * difference vs the standard pmd_none_or_clear_bad, the result is
> - * undefined so behaving like if the pmd was none is safe (because it
> - * can return none anyway). The compiler level barrier() is critically
> - * important to compute the two checks atomically on the same pmdval.
> - */
> -static inline int pmd_none_or_trans_huge_or_clear_bad(pmd_t *pmd)
> -{
> -	/* depend on compiler for an atomic pmd read */
> -	pmd_t pmdval = *pmd;
> -	/*
> -	 * The barrier will stabilize the pmdval in a register or on
> -	 * the stack so that it will stop changing under the code.
> -	 */
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -	barrier();
> -#endif
> -	if (pmd_none(pmdval))
> -		return 1;
> -	if (unlikely(pmd_bad(pmdval))) {
> -		if (!pmd_trans_huge(pmdval))
> -			pmd_clear_bad(pmd);
> -		return 1;
> -	}
> -	return 0;
> -}
> -
> -/*
> - * This is a noop if Transparent Hugepage Support is not built into
> - * the kernel. Otherwise it is equivalent to
> - * pmd_none_or_trans_huge_or_clear_bad(), and shall only be called in
> - * places that already verified the pmd is not none and they want to
> - * walk ptes while holding the mmap sem in read mode (write mode don't
> - * need this). If THP is not enabled, the pmd can't go away under the
> - * code even if MADV_DONTNEED runs, but if THP is enabled we need to
> - * run a pmd_trans_unstable before walking the ptes after
> - * split_huge_page_pmd returns (because it may have run when the pmd
> - * become null, but then a page fault can map in a THP and not a
> - * regular page).
> - */
> -static inline int pmd_trans_unstable(pmd_t *pmd)
> -{
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -	return pmd_none_or_trans_huge_or_clear_bad(pmd);
> -#else
> -	return 0;
> -#endif
> -}
> -
>  #endif /* !__ASSEMBLY__ */
>  
>  #endif /* _ASM_GENERIC_PGTABLE_H */
> 
> 

-- 
Cheers,
Stephen Rothwell                    sfr@...b.auug.org.au

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