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    Subject[PATCH 3.11 70/94] mm: Wait for THP migrations to complete during NUMA hinting faults
    Date
    3.11-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Mel Gorman <mgorman@suse.de>

    commit 42836f5f8baa33085f547098b74aa98991ee9216 upstream.

    The locking for migrating THP is unusual. While normal page migration
    prevents parallel accesses using a migration PTE, THP migration relies on
    a combination of the page_table_lock, the page lock and the existance of
    the NUMA hinting PTE to guarantee safety but there is a bug in the scheme.

    If a THP page is currently being migrated and another thread traps a
    fault on the same page it checks if the page is misplaced. If it is not,
    then pmd_numa is cleared. The problem is that it checks if the page is
    misplaced without holding the page lock meaning that the racing thread
    can be migrating the THP when the second thread clears the NUMA bit
    and faults a stale page.

    This patch checks if the page is potentially being migrated and stalls
    using the lock_page if it is potentially being migrated before checking
    if the page is misplaced or not.

    Signed-off-by: Mel Gorman <mgorman@suse.de>
    Reviewed-by: Rik van Riel <riel@redhat.com>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
    Signed-off-by: Peter Zijlstra <peterz@infradead.org>
    Link: http://lkml.kernel.org/r/1381141781-10992-6-git-send-email-mgorman@suse.de
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    mm/huge_memory.c | 23 ++++++++++++++++-------
    1 file changed, 16 insertions(+), 7 deletions(-)

    --- a/mm/huge_memory.c
    +++ b/mm/huge_memory.c
    @@ -1307,13 +1307,14 @@ int do_huge_pmd_numa_page(struct mm_stru
    if (current_nid == numa_node_id())
    count_vm_numa_event(NUMA_HINT_FAULTS_LOCAL);

    - target_nid = mpol_misplaced(page, vma, haddr);
    - if (target_nid == -1) {
    - put_page(page);
    - goto clear_pmdnuma;
    - }
    + /*
    + * Acquire the page lock to serialise THP migrations but avoid dropping
    + * page_table_lock if at all possible
    + */
    + if (trylock_page(page))
    + goto got_lock;

    - /* Acquire the page lock to serialise THP migrations */
    + /* Serialise against migrationa and check placement check placement */
    spin_unlock(&mm->page_table_lock);
    lock_page(page);

    @@ -1324,9 +1325,17 @@ int do_huge_pmd_numa_page(struct mm_stru
    put_page(page);
    goto out_unlock;
    }
    - spin_unlock(&mm->page_table_lock);
    +
    +got_lock:
    + target_nid = mpol_misplaced(page, vma, haddr);
    + if (target_nid == -1) {
    + unlock_page(page);
    + put_page(page);
    + goto clear_pmdnuma;
    + }

    /* Migrate the THP to the requested node */
    + spin_unlock(&mm->page_table_lock);
    migrated = migrate_misplaced_transhuge_page(mm, vma,
    pmdp, pmd, addr, page, target_nid);
    if (!migrated)



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