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SubjectRe: [PATCH 3/3] Guarantee that COW faults for a process that called mmap(MAP_PRIVATE) on hugetlbfs will succeed
FromAdam Litke <>
DateWed, 14 May 2008 15:55:25 -0500
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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