Re: [Xenomai] [PATCH v2] ipipe x86 mm: handle huge pages in memory pinning

[Philippe Gerum <rpm@xenomai.org>]

On 01/27/2016 02:41 PM, Henning Schild wrote:
> In 4.1 huge page mapping of io memory was introduced, enable ipipe to
> handle that when pinning kernel memory.
> 
> change that introduced the feature
> 0f616be120c632c818faaea9adcb8f05a7a8601f
> 
> Signed-off-by: Henning Schild <henning.schild@siemens.com>
> ---
>  arch/x86/mm/fault.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index fd5bbcc..ca1e75b 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -211,11 +211,15 @@ static inline pmd_t *vmalloc_sync_one(pgd_t *pgd, unsigned long address)
>  	pud_k = pud_offset(pgd_k, address);
>  	if (!pud_present(*pud_k))
>  		return NULL;
> +	if (pud_large(*pud))
> +		return pud_k;
>  
>  	pmd = pmd_offset(pud, address);
>  	pmd_k = pmd_offset(pud_k, address);
>  	if (!pmd_present(*pmd_k))
>  		return NULL;
> +	if (pmd_large(*pmd))
> +		return pmd_k;
>  
>  	if (!pmd_present(*pmd))
>  		set_pmd(pmd, *pmd_k);
> @@ -400,6 +404,8 @@ static inline int vmalloc_sync_one(pgd_t *pgd, unsigned long address)
>  
>  	if (pud_none(*pud) || pud_page_vaddr(*pud) != pud_page_vaddr(*pud_ref))
>  		BUG();
> +	if (pud_large(*pud))
> +		return 0;
>  
>  	pmd = pmd_offset(pud, address);
>  	pmd_ref = pmd_offset(pud_ref, address);
> @@ -408,6 +414,8 @@ static inline int vmalloc_sync_one(pgd_t *pgd, unsigned long address)
>  
>  	if (pmd_none(*pmd) || pmd_page(*pmd) != pmd_page(*pmd_ref))
>  		BUG();
> +	if (pmd_large(*pmd))
> +		return 0;
>  
>  	pte_ref = pte_offset_kernel(pmd_ref, address);
>  	if (!pte_present(*pte_ref))
> 

I'm confused. Assuming the purpose of that patch is to exclude huge I/O
mappings from pte pinning, why does the changes to the x86_32 version of
the vmalloc_sync_one() helper actually prevent such pinning, while the
x86_64 version does not?

Could you explain the logic behind the changes?

-- 
Philippe.