Subject: Re: [PATCH] powerpc/mm/hash: Clear the invalid slot information correctly

From: Anshuman Khandual <[email-masked]>
Date: Fri, 19 Feb 2016 11:23:10 +0530

On 02/18/2016 10:14 PM, Aneesh Kumar K.V wrote:
> We can get a hash pte fault with 4k base page size and find the pte
> already inserted with 64K base page size. In that case we need to clear

Can you please elaborate on this ? What are those situations when we
have 64K base page size on the PTE but we had inserted HPTE with base
page size as 4K ?

> the existing slot information from the old pte. Fix this correctly
> 
> With THP, we also clear the slot information with respect to all
> the 64K hash pte mapping that 16MB page. They are all invalid
> now. This make sure we don't find the slot valid when we fault with
> 4k base page size. Finding the slot valid should not result in any wrong
> behavior because we do check again in hash page table for the validity.
> But we can avoid that check completely.

Makes sense.

> 
> Fixes: a43c0eb8364c022 ("powerpc/mm: Convert 4k hash insert to C")
> 
> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
> ---
>  arch/powerpc/mm/hash64_4k.c       |  2 +-
>  arch/powerpc/mm/hash64_64k.c      | 12 +++++++++---
>  arch/powerpc/mm/hugepage-hash64.c |  7 ++++++-
>  3 files changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/powerpc/mm/hash64_4k.c b/arch/powerpc/mm/hash64_4k.c
> index e7c04542ba62..e3e76b929f33 100644
> --- a/arch/powerpc/mm/hash64_4k.c
> +++ b/arch/powerpc/mm/hash64_4k.c
> @@ -106,7 +106,7 @@ repeat:
>  			}
>  		}
>  		/*
> -		 * Hypervisor failure. Restore old pmd and return -1
> +		 * Hypervisor failure. Restore old pte and return -1

This change is not relevant here. Should be a separate patch.

>  		 * similar to __hash_page_*
>  		 */
>  		if (unlikely(slot == -2)) {
> diff --git a/arch/powerpc/mm/hash64_64k.c b/arch/powerpc/mm/hash64_64k.c
> index 0762c1e08c88..b3895720edb0 100644
> --- a/arch/powerpc/mm/hash64_64k.c
> +++ b/arch/powerpc/mm/hash64_64k.c
> @@ -111,7 +111,13 @@ int __hash_page_4K(unsigned long ea, unsigned long access, unsigned long vsid,
>  	 */
>  	if (!(old_pte & _PAGE_COMBO)) {
>  		flush_hash_page(vpn, rpte, MMU_PAGE_64K, ssize, flags);
> -		old_pte &= ~_PAGE_HASHPTE | _PAGE_F_GIX | _PAGE_F_SECOND;
> +		/*
> +		 * clear the old slot details from the old and new pte.
> +		 * On hash insert failure we use old pte value and we don't
> +		 * want slot information there if we have a insert failure.
> +		 */
> +		old_pte &= ~(_PAGE_HASHPTE | _PAGE_F_GIX | _PAGE_F_SECOND);
> +		new_pte &= ~(_PAGE_HASHPTE | _PAGE_F_GIX | _PAGE_F_SECOND);

But why we need clear the bits on new_pte as well ?

>  		goto htab_insert_hpte;
>  	}
>  	/*
> @@ -182,7 +188,7 @@ repeat:
>  		}
>  	}
>  	/*
> -	 * Hypervisor failure. Restore old pmd and return -1
> +	 * Hypervisor failure. Restore old pte and return -1

This change is not relevant here. Should be a separate patch.

>  	 * similar to __hash_page_*
>  	 */
>  	if (unlikely(slot == -2)) {
> @@ -305,7 +311,7 @@ repeat:
>  			}
>  		}
>  		/*
> -		 * Hypervisor failure. Restore old pmd and return -1
> +		 * Hypervisor failure. Restore old pte and return -1
>  		 * similar to __hash_page_*

Ditto.