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    SubjectRe: [rfc 5/5] mincore: transparent huge page support
    On Wed, Mar 24, 2010 at 11:48:58PM +0100, Andrea Arcangeli wrote:
    > On Tue, Mar 23, 2010 at 03:35:02PM +0100, Johannes Weiner wrote:
    > > +static int mincore_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
    > > + unsigned long addr, unsigned long end,
    > > + unsigned char *vec)
    > > +{
    > > + int huge = 0;
    > > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
    > > + spin_lock(&vma->vm_mm->page_table_lock);
    > > + if (likely(pmd_trans_huge(*pmd))) {
    > > + huge = !pmd_trans_splitting(*pmd);
    >
    > Under mmap_sem (read or write) a hugepage can't materialize under
    > us. So here the pmd_trans_huge can be lockless and run _before_ taking
    > the page_table_lock. That's the invariant I used to keep identical
    > performance for all fast paths.

    Wait, there _is_ an unlocked fast-path pmd_trans_huge()
    in mincore_pmd_range(), maybe you missed it?

    This function is never called if the pmd is not huge.

    So the above is the _second check_ under lock to get a stable
    read on the entry that could be splitting or already have been
    split while we checked locklessly.


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