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    Subject[PATCH] mm: Improve documentation of page_order v2
    Developers occasionally try and optimise PFN scanners by using page_order
    but miss that in general it requires zone->lock. This has happened twice for
    compaction.c and rejected both times. This patch clarifies the documentation
    of page_order and adds a note to compaction.c why page_order is not used.

    [lauraa@codeaurora.org: Corrected a page_zone(page)->lock reference]
    Signed-off-by: Mel Gorman <mgorman@suse.de>
    Acked-by: Rafael Aquini <aquini@redhat.com>
    Acked-by: Minchan Kim <minchan@kernel.org>
    ---
    mm/compaction.c | 5 ++++-
    mm/internal.h | 8 +++++---
    2 files changed, 9 insertions(+), 4 deletions(-)

    diff --git a/mm/compaction.c b/mm/compaction.c
    index f58bcd0..f91d26b 100644
    --- a/mm/compaction.c
    +++ b/mm/compaction.c
    @@ -522,7 +522,10 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
    if (!isolation_suitable(cc, page))
    goto next_pageblock;

    - /* Skip if free */
    + /*
    + * Skip if free. page_order cannot be used without zone->lock
    + * as nothing prevents parallel allocations or buddy merging.
    + */
    if (PageBuddy(page))
    continue;

    diff --git a/mm/internal.h b/mm/internal.h
    index 684f7aa..09cd8be 100644
    --- a/mm/internal.h
    +++ b/mm/internal.h
    @@ -144,9 +144,11 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
    #endif

    /*
    - * function for dealing with page's order in buddy system.
    - * zone->lock is already acquired when we use these.
    - * So, we don't need atomic page->flags operations here.
    + * This functions returns the order of a free page in the buddy system. In
    + * general, page_zone(page)->lock must be held by the caller to prevent the
    + * page being allocated in parallel and returning garbage as the order. If the
    + * caller does not hold page_zone(page)->lock, they must guarantee that the
    + * page cannot be allocated or merged in parallel.
    */
    static inline unsigned long page_order(struct page *page)
    {


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