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    SubjectRe: [PATCH 27/50] mm: numa: Scan pages with elevated page_mapcount
    From
    Hillo Mel

    On Tue, Sep 10, 2013 at 5:32 PM, Mel Gorman <mgorman@suse.de> wrote:
    > Currently automatic NUMA balancing is unable to distinguish between false
    > shared versus private pages except by ignoring pages with an elevated
    > page_mapcount entirely. This avoids shared pages bouncing between the
    > nodes whose task is using them but that is ignored quite a lot of data.
    >
    > This patch kicks away the training wheels in preparation for adding support
    > for identifying shared/private pages is now in place. The ordering is so
    > that the impact of the shared/private detection can be easily measured. Note
    > that the patch does not migrate shared, file-backed within vmas marked
    > VM_EXEC as these are generally shared library pages. Migrating such pages
    > is not beneficial as there is an expectation they are read-shared between
    > caches and iTLB and iCache pressure is generally low.
    >
    > Signed-off-by: Mel Gorman <mgorman@suse.de>
    > ---
    [...]
    > @@ -1658,13 +1660,6 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
    > int page_lru = page_is_file_cache(page);
    >
    > /*
    > - * Don't migrate pages that are mapped in multiple processes.
    > - * TODO: Handle false sharing detection instead of this hammer
    > - */
    > - if (page_mapcount(page) != 1)
    > - goto out_dropref;
    > -
    Is there rmap walk when migrating THP?

    > - /*
    > * Rate-limit the amount of data that is being migrated to a node.
    > * Optimal placement is no good if the memory bus is saturated and
    > * all the time is being spent migrating!


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