Subject: RE: [PATCH V3 1/2] mm: move FAULT_AROUND_ORDER to arch/

From: "Kirill A. Shutemov" <[email-masked]>
Date: Mon, 28 Apr 2014 12:36:06 +0300 (EEST)

Madhavan Srinivasan wrote:
> Kirill A. Shutemov with 8c6e50b029 commit introduced
> vm_ops->map_pages() for mapping easy accessible pages around
> fault address in hope to reduce number of minor page faults.
> 
> This patch creates infrastructure to modify the FAULT_AROUND_ORDER
> value using mm/Kconfig. This will enable architecture maintainers
> to decide on suitable FAULT_AROUND_ORDER value based on
> performance data for that architecture. Patch also defaults
> FAULT_AROUND_ORDER Kconfig element to 4.
> 
> Signed-off-by: Madhavan Srinivasan <[email-masked]>
> ---
>  mm/Kconfig  |    8 ++++++++
>  mm/memory.c |   11 ++++-------
>  2 files changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/Kconfig b/mm/Kconfig
> index ebe5880..c7fc4f1 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -176,6 +176,14 @@ config MOVABLE_NODE
>  config HAVE_BOOTMEM_INFO_NODE
>  	def_bool n
>  
> +#
> +# Fault around order is a control knob to decide the fault around pages.
> +# Default value is set to 4 , but the arch can override it as desired.
> +#
> +config FAULT_AROUND_ORDER
> +	int
> +	default	4
> +
>  # eventually, we can have this option just 'select SPARSEMEM'
>  config MEMORY_HOTPLUG
>  	bool "Allow for memory hot-add"
> diff --git a/mm/memory.c b/mm/memory.c
> index d0f0bef..457436d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3382,11 +3382,9 @@ void do_set_pte(struct vm_area_struct *vma, unsigned long address,
>  	update_mmu_cache(vma, address, pte);
>  }
>  
> -#define FAULT_AROUND_ORDER 4
> +unsigned int fault_around_order = CONFIG_FAULT_AROUND_ORDER;
>  
>  #ifdef CONFIG_DEBUG_FS
> -static unsigned int fault_around_order = FAULT_AROUND_ORDER;
> -
>  static int fault_around_order_get(void *data, u64 *val)
>  {
>  	*val = fault_around_order;
> @@ -3395,7 +3393,6 @@ static int fault_around_order_get(void *data, u64 *val)
>  
>  static int fault_around_order_set(void *data, u64 val)
>  {
> -	BUILD_BUG_ON((1UL << FAULT_AROUND_ORDER) > PTRS_PER_PTE);
>  	if (1UL << val > PTRS_PER_PTE)
>  		return -EINVAL;
>  	fault_around_order = val;
> @@ -3430,14 +3427,14 @@ static inline unsigned long fault_around_pages(void)
>  {
>  	unsigned long nr_pages;
>  
> -	nr_pages = 1UL << FAULT_AROUND_ORDER;
> +	nr_pages = 1UL << fault_around_order;
>  	BUILD_BUG_ON(nr_pages > PTRS_PER_PTE);

This BUILD_BUG_ON() doesn't make sense any more since compiler usually can't
prove anything about extern variable.

Drop it or change to VM_BUG_ON() or something.

>  	return nr_pages;
>  }
>  
>  static inline unsigned long fault_around_mask(void)
>  {
> -	return ~((1UL << (PAGE_SHIFT + FAULT_AROUND_ORDER)) - 1);
> +	return ~((1UL << (PAGE_SHIFT + fault_around_order)) - 1);

fault_around_pages() and fault_around_mask() should be moved outside of
#ifdef CONFIG_DEBUG_FS and consolidated since they are practically identical
both branches after the changes.

>  }
>  #endif
>  
> @@ -3495,7 +3492,7 @@ static int do_read_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>  	 * if page by the offset is not ready to be mapped (cold cache or
>  	 * something).
>  	 */
> -	if (vma->vm_ops->map_pages) {
> +	if ((vma->vm_ops->map_pages) && (fault_around_order)) {

Way too many parentheses.

>  		pte = pte_offset_map_lock(mm, pmd, address, &ptl);
>  		do_fault_around(vma, address, pte, pgoff, flags);
>  		if (!pte_same(*pte, orig_pte))
-- 
 Kirill A. Shutemov