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    Subject[PATCH v4 4/4] mm/page_alloc: restrict max order of merging on isolated pageblock
    Date
    Current pageblock isolation logic could isolate each pageblock
    individually. This causes freepage accounting problem if freepage with
    pageblock order on isolate pageblock is merged with other freepage on
    normal pageblock. We can prevent merging by restricting max order of
    merging to pageblock order if freepage is on isolate pageblock.

    Side-effect of this change is that there could be non-merged buddy
    freepage even if finishing pageblock isolation, because undoing pageblock
    isolation is just to move freepage from isolate buddy list to normal buddy
    list rather than to consider merging. But, I think it doesn't matter
    because 1) almost allocation request are for equal or below pageblock
    order, 2) caller of pageblock isolation will use this freepage so
    freepage will split in any case and 3) merge would happen soon after
    some alloc/free on this and buddy pageblock.

    Cc: <stable@vger.kernel.org>
    Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    ---
    mm/page_alloc.c | 15 ++++++++++++---
    1 file changed, 12 insertions(+), 3 deletions(-)

    diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    index 433f92c..3ec58db 100644
    --- a/mm/page_alloc.c
    +++ b/mm/page_alloc.c
    @@ -571,6 +571,7 @@ static inline void __free_one_page(struct page *page,
    unsigned long combined_idx;
    unsigned long uninitialized_var(buddy_idx);
    struct page *buddy;
    + int max_order = MAX_ORDER;

    VM_BUG_ON(!zone_is_initialized(zone));

    @@ -582,18 +583,26 @@ static inline void __free_one_page(struct page *page,
    if (unlikely(has_isolate_pageblock(zone) ||
    is_migrate_isolate(migratetype))) {
    migratetype = get_pfnblock_migratetype(page, pfn);
    - if (is_migrate_isolate(migratetype))
    + if (is_migrate_isolate(migratetype)) {
    + /*
    + * We restrict max order of merging to prevent merge
    + * between freepages on isolate pageblock and normal
    + * pageblock. Without this, pageblock isolation
    + * could cause incorrect freepage accounting.
    + */
    + max_order = min(MAX_ORDER, pageblock_order + 1);
    goto skip_counting;
    + }
    }
    __mod_zone_freepage_state(zone, 1 << order, migratetype);

    skip_counting:
    - page_idx = pfn & ((1 << MAX_ORDER) - 1);
    + page_idx = pfn & ((1 << max_order) - 1);

    VM_BUG_ON_PAGE(page_idx & ((1 << order) - 1), page);
    VM_BUG_ON_PAGE(bad_range(zone, page), page);

    - while (order < MAX_ORDER-1) {
    + while (order < max_order - 1) {
    buddy_idx = __find_buddy_index(page_idx, order);
    buddy = page + (buddy_idx - page_idx);
    if (!page_is_buddy(page, buddy, order))
    --
    1.7.9.5


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