Subject: Re: [PATCH -V5 21/25] powerpc: Handle hugepage in perf callchain

From: David Gibson <[email-masked]>
Date: Fri, 12 Apr 2013 11:34:49 +1000

On Thu, Apr 04, 2013 at 11:27:59AM +0530, Aneesh Kumar K.V wrote:
> From: "Aneesh Kumar K.V" <[email-masked]>
> 
> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
> ---
>  arch/powerpc/perf/callchain.c |   32 +++++++++++++++++++++-----------
>  1 file changed, 21 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/powerpc/perf/callchain.c b/arch/powerpc/perf/callchain.c
> index 578cac7..99262ce 100644
> --- a/arch/powerpc/perf/callchain.c
> +++ b/arch/powerpc/perf/callchain.c
> @@ -115,7 +115,7 @@ static int read_user_stack_slow(void __user *ptr, void *ret, int nb)
>  {
>  	pgd_t *pgdir;
>  	pte_t *ptep, pte;
> -	unsigned shift;
> +	unsigned shift, hugepage;
>  	unsigned long addr = (unsigned long) ptr;
>  	unsigned long offset;
>  	unsigned long pfn;
> @@ -125,20 +125,30 @@ static int read_user_stack_slow(void __user *ptr, void *ret, int nb)
>  	if (!pgdir)
>  		return -EFAULT;
>  
> -	ptep = find_linux_pte_or_hugepte(pgdir, addr, &shift, NULL);
> +	ptep = find_linux_pte_or_hugepte(pgdir, addr, &shift, &hugepage);

So, this patch pretty much demonstrates that your earlier patch adding
the optional hugepage argument and making the existing callers pass
NULL was broken.

Any code which calls this function and doesn't use and handle the
hugepage return value is horribly broken, so permitting the hugepage
parameter to be optional is itself broken.

I think instead you need to have an early patch that replaces
find_linux_pte_or_hugepte with a new, more abstracted interface, so
that code using it will remain correct when hugepage PMDs become
possible.

>  	if (!shift)
>  		shift = PAGE_SHIFT;
>  
> -	/* align address to page boundary */
> -	offset = addr & ((1UL << shift) - 1);
> -	addr -= offset;
> -
> -	if (ptep == NULL)
> -		return -EFAULT;
> -	pte = *ptep;
> -	if (!pte_present(pte) || !(pte_val(pte) & _PAGE_USER))
> +	if (!ptep)
>  		return -EFAULT;
> -	pfn = pte_pfn(pte);
> +
> +	if (hugepage) {
> +		pmd_t pmd = *(pmd_t *)ptep;
> +		shift = mmu_psize_defs[MMU_PAGE_16M].shift;
> +		offset = addr & ((1UL << shift) - 1);
> +
> +		if (!pmd_large(pmd) || !(pmd_val(pmd) & PMD_HUGE_USER))
> +			return -EFAULT;
> +		pfn = pmd_pfn(pmd);
> +	} else {
> +		offset = addr & ((1UL << shift) - 1);
> +
> +		pte = *ptep;
> +		if (!pte_present(pte) || !(pte_val(pte) & _PAGE_USER))
> +			return -EFAULT;
> +		pfn = pte_pfn(pte);
> +	}
> +
>  	if (!page_is_ram(pfn))
>  		return -EFAULT;
>  

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson