Subject: Re: [PATCH] powerpc: thp: Add write barrier after updating the valid bit

From: Benjamin Herrenschmidt <[email-masked]>
Date: Tue, 22 Jul 2014 15:27:42 +1000

On Tue, 2014-07-15 at 20:22 +0530, Aneesh Kumar K.V wrote:
> With hugepages, we store the hpte valid information in the pte page
> whose address is stored in the second half of the PMD. Use a
> write barrier to make sure that clearing pmd busy bit and updating
> hpte valid info are ordered properly.
> 
> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
> ---
>  arch/powerpc/include/asm/pgtable-ppc64.h | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/pgtable-ppc64.h b/arch/powerpc/include/asm/pgtable-ppc64.h
> index eb9261024f51..558beb760062 100644
> --- a/arch/powerpc/include/asm/pgtable-ppc64.h
> +++ b/arch/powerpc/include/asm/pgtable-ppc64.h
> @@ -394,6 +394,12 @@ static inline void mark_hpte_slot_valid(unsigned char *hpte_slot_array,
>  					unsigned int index, unsigned int hidx)
>  {
>  	hpte_slot_array[index] = hidx << 4 | 0x1 << 3;
> +	/*
> +	 * The hpte valid is stored in the pgtable whose address is in the
> +	 * second half of the PMD. Order this against clearing of the busy bit in
> +	 * huge pmd.
> +	 */
> +	smp_wmb();
>  }

A better place for this would be right before the last write to the PMD
(that's also clearing BUSY) in __hash_page_thp(). Basically, it's the
normal lock ordering that's missing here, nothing specific to
mark_hpte_slot_valid() but instead, any state relative to the BUSY bit
in the PMD (including the actual hash writes in update_pp etc...)

>  struct page *realmode_pfn_to_page(unsigned long pfn);