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

From: Mel Gorman
Date: Fri May 16 2008 - 08:15:27 EST


On (14/05/08 15:55), Adam Litke didst pronounce:
> 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;
> };
>

That won't fit into a void * on 32 bit. The use of a pointer to store
values like this is a little ugly but it's confined to the helpers whose
naming makes it obvious what is going on.

It would be done with bit-fields and the like but I don't think it helps
the readability a whole lot.

> > 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
>

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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