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    SubjectRe: Race in new page migration code?
    On Tue, 17 Jan 2006, Hugh Dickins wrote:

    > Of course, page_mapcount may go up an instant later; but equally,
    > another vma may get added to the prio_tree an instant later.

    Right. The check does not have to be race free.

    > It may be that, after much argument to and fro, the whole function will
    > just reduce to checking "page_mapcount(page) == 1": if so, then I think
    > you can go back to inlining it literally.

    Oh. I love simplicity.... Sadly, one sometimes has these complicated
    notions in ones brain. Thanks.


    Simplify migrate_page_add after feedback from Hugh.

    Signed-off-by: Christoph Lameter <clameter@sgi.com>

    Index: linux-2.6.15/mm/mempolicy.c
    ===================================================================
    --- linux-2.6.15.orig/mm/mempolicy.c 2006-01-14 10:56:28.000000000 -0800
    +++ linux-2.6.15/mm/mempolicy.c 2006-01-17 12:12:42.000000000 -0800
    @@ -527,42 +527,13 @@ long do_get_mempolicy(int *policy, nodem
    * page migration
    */

    -/* Check if we are the only process mapping the page in question */
    -static inline int single_mm_mapping(struct mm_struct *mm,
    - struct address_space *mapping)
    -{
    - struct vm_area_struct *vma;
    - struct prio_tree_iter iter;
    - int rc = 1;
    -
    - spin_lock(&mapping->i_mmap_lock);
    - vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, 0, ULONG_MAX)
    - if (mm != vma->vm_mm) {
    - rc = 0;
    - goto out;
    - }
    - list_for_each_entry(vma, &mapping->i_mmap_nonlinear, shared.vm_set.list)
    - if (mm != vma->vm_mm) {
    - rc = 0;
    - goto out;
    - }
    -out:
    - spin_unlock(&mapping->i_mmap_lock);
    - return rc;
    -}
    -
    -/*
    - * Add a page to be migrated to the pagelist
    - */
    static void migrate_page_add(struct vm_area_struct *vma,
    struct page *page, struct list_head *pagelist, unsigned long flags)
    {
    /*
    - * Avoid migrating a page that is shared by others and not writable.
    + * Avoid migrating a page that is shared with others.
    */
    - if ((flags & MPOL_MF_MOVE_ALL) || !page->mapping || PageAnon(page) ||
    - mapping_writably_mapped(page->mapping) ||
    - single_mm_mapping(vma->vm_mm, page->mapping))
    + if ((flags & MPOL_MF_MOVE_ALL) || page_mapcount(page) ==1)
    if (isolate_lru_page(page) == 1)
    list_add(&page->lru, pagelist);
    }
    -
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