Re: [PATCH 3/3] Guarantee that COW faults for a process that called mmap(MAP_PRIVATE) on hugetlbfs will succeed

[Adam Litke <agl@us.ibm.com>]

On Wed, 2008-05-07 at 20:39 +0100, Mel Gorman wrote:

> diff -rup -X /usr/src/patchset-0.6/bin//dontdiff linux-2.6.25-mm1-0020-map_private_reserve/mm/hugetlb.c linux-2.6.25-mm1-0030-reliable_parent_faults/mm/hugetlb.c
> --- linux-2.6.25-mm1-0020-map_private_reserve/mm/hugetlb.c	2008-05-07 18:39:34.000000000 +0100
> +++ linux-2.6.25-mm1-0030-reliable_parent_faults/mm/hugetlb.c	2008-05-07 20:05:18.000000000 +0100
> @@ -40,6 +40,9 @@ static int hugetlb_next_nid;
>   */
>  static DEFINE_SPINLOCK(hugetlb_lock);
> 
> +#define HPAGE_RESV_OWNER    (1UL << (BITS_PER_LONG - 1))
> +#define HPAGE_RESV_UNMAPPED (1UL << (BITS_PER_LONG - 2))
> +#define HPAGE_RESV_MASK (HPAGE_RESV_OWNER | HPAGE_RESV_UNMAPPED)
>  /*
>   * These three helpers are used to track how many pages are reserved for
>   * faults in a MAP_PRIVATE mapping. Only the process that called mmap()
> @@ -49,20 +52,23 @@ static unsigned long vma_resv_huge_pages
>  {
>  	VM_BUG_ON(!is_vm_hugetlb_page(vma));
>  	if (!(vma->vm_flags & VM_SHARED))
> -		return (unsigned long)vma->vm_private_data;
> +		return (unsigned long)vma->vm_private_data & ~HPAGE_RESV_MASK;
>  	return 0;
>  }

Ick.  Though I don't really have a suggestion on how to improve it
unless a half-word is enough room to contain the reservation.  In that
case we could create a structure which would make this much clearer.

struct hugetlb_vma_reservation {
	unsigned int flags;
	unsigned int resv;
};

>  static void adjust_vma_resv_huge_pages(struct vm_area_struct *vma, int delta)
>  {
>  	unsigned long reserve;
> +	unsigned long flags;
>  	VM_BUG_ON(vma->vm_flags & VM_SHARED);
>  	VM_BUG_ON(!is_vm_hugetlb_page(vma));
> 
>  	reserve = (unsigned long)vma->vm_private_data + delta;
> -	vma->vm_private_data = (void *)reserve;
> +	flags = (unsigned long)vma->vm_private_data & HPAGE_RESV_MASK;
> +	vma->vm_private_data = (void *)(reserve | flags);
>  }
> 
> +/* Reset counters to 0 and clear all HPAGE_RESV_* flags */
>  void reset_vma_resv_huge_pages(struct vm_area_struct *vma)
>  {
>  	VM_BUG_ON(!is_vm_hugetlb_page(vma));
> @@ -73,10 +79,27 @@ void reset_vma_resv_huge_pages(struct vm
>  static void set_vma_resv_huge_pages(struct vm_area_struct *vma,
>  							unsigned long reserve)
>  {
> +	unsigned long flags;
> +
>  	VM_BUG_ON(!is_vm_hugetlb_page(vma));
>  	VM_BUG_ON(vma->vm_flags & VM_SHARED);
> 
> -	vma->vm_private_data = (void *)reserve;
> +	flags = (unsigned long)vma->vm_private_data & HPAGE_RESV_MASK;
> +	vma->vm_private_data = (void *)(reserve | flags);
> +}
> +
> +static void set_vma_resv_flags(struct vm_area_struct *vma, unsigned long flags)
> +{
> +	unsigned long reserveflags = (unsigned long)vma->vm_private_data;
> +	VM_BUG_ON(!is_vm_hugetlb_page(vma));
> +	reserveflags |= flags;
> +	vma->vm_private_data = (void *)reserveflags;
> +}
> +
> +static int is_vma_resv_set(struct vm_area_struct *vma, unsigned long flag)
> +{
> +	VM_BUG_ON(!is_vm_hugetlb_page(vma));
> +	return ((unsigned long)vma->vm_private_data & flag) != 0;
>  }
> 
>  static void clear_huge_page(struct page *page, unsigned long addr)
> @@ -139,7 +162,7 @@ static void decrement_hugepage_resv_vma(
>  		 * Only the process that called mmap() has reserves for
>  		 * private mappings.
>  		 */
> -		if (vma_resv_huge_pages(vma)) {
> +		if (is_vma_resv_set(vma, HPAGE_RESV_OWNER)) {
>  			resv_huge_pages--;
>  			adjust_vma_resv_huge_pages(vma, -1);
>  		}

-- 
Adam Litke - (agl at us.ibm.com)
IBM Linux Technology Center

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