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    SubjectRe: [PATCH 2/2] Make is_mem_section_removable more conformable with offlining code v3
    On Fri, Sep 03, 2010 at 07:42:13PM +0800, Michal Hocko wrote:

    > +/*
    > + * A free or LRU pages block are removable
    > + * Do not use MIGRATE_MOVABLE because it can be insufficient and
    > + * other MIGRATE types are tricky.
    > + * Do not hold zone->lock as this is used from user space by the
    > + * sysfs interface.
    > + */
    > +bool is_page_removable(struct page *page)
    > +{
    > + int page_block = 1 << pageblock_order;
    > +
    > + /* All pages from the MOVABLE zone are movable */
    > + if (zone_idx(page_zone(page)) == ZONE_MOVABLE)
    > + return true;
    > +
    > + while (page_block > 0) {
    > + int order = 0;
    > +
    > + if (pfn_valid_within(page_to_pfn(page))) {
    > + if (!page_count(page) && PageBuddy(page)) {

    PageBuddy() is true only for the head page and false for all tail
    pages. So when is_page_removable() is given a random 4k page
    (get_any_page() will exactly do that), the above test is not enough.

    It's recommended to reuse is_free_buddy_page(). (Need to do some
    cleanup work first: remove the "#ifdef CONFIG_MEMORY_FAILURE" and
    abstract out an __is_free_buddy_page() that takes no lock.)

    > @@ -5277,14 +5277,11 @@ int set_migratetype_isolate(struct page *page)
    > struct memory_isolate_notify arg;
    > int notifier_ret;
    > int ret = -EBUSY;
    > - int zone_idx;
    >
    > zone = page_zone(page);
    > - zone_idx = zone_idx(zone);
    >
    > spin_lock_irqsave(&zone->lock, flags);
    > - if (get_pageblock_migratetype(page) == MIGRATE_MOVABLE ||
    > - zone_idx == ZONE_MOVABLE) {
    > + if (is_page_removable(page)) {
    > ret = 0;
    > goto out;

    The above check only applies to the first page in the page block.
    The following "if (!page_count(curr_page) || PageLRU(curr_page))"
    check in the same function should be converted too (and that's another
    reason to use __is_free_buddy_page(): it will be tested for every 4k
    pages, including both the head and tail pages).

    Thanks,
    Fengguang


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