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    Subject[PATCH v2 04/11] memcg: disable softirq in lock_page_cgroup()
    Date
    If pages are being migrated from a memcg, then updates to that
    memcg's page statistics are protected by grabbing a bit spin lock
    using lock_page_cgroup(). In an upcoming commit memcg dirty page
    accounting will be updating memcg page accounting (specifically:
    num writeback pages) from softirq. Avoid a deadlocking nested
    spin lock attempt by disabling softirq on the local processor
    when grabbing the page_cgroup bit_spin_lock in lock_page_cgroup().
    This avoids the following deadlock:
    statistic
    CPU 0 CPU 1
    inc_file_mapped
    rcu_read_lock
    start move
    synchronize_rcu
    lock_page_cgroup
    softirq
    test_clear_page_writeback
    mem_cgroup_dec_page_stat(NR_WRITEBACK)
    rcu_read_lock
    lock_page_cgroup /* deadlock */
    unlock_page_cgroup
    rcu_read_unlock
    unlock_page_cgroup
    rcu_read_unlock

    By disabling softirq in lock_page_cgroup, nested calls are avoided.
    The softirq would be delayed until after inc_file_mapped enables
    softirq when calling unlock_page_cgroup().

    The normal, fast path, of memcg page stat updates typically
    does not need to call lock_page_cgroup(), so this change does
    not affect the performance of the common case page accounting.

    Signed-off-by: Andrea Righi <arighi@develer.com>
    Signed-off-by: Greg Thelen <gthelen@google.com>
    ---
    include/linux/page_cgroup.h | 6 ++++++
    1 files changed, 6 insertions(+), 0 deletions(-)

    diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
    index b59c298..0585546 100644
    --- a/include/linux/page_cgroup.h
    +++ b/include/linux/page_cgroup.h
    @@ -3,6 +3,8 @@

    #ifdef CONFIG_CGROUP_MEM_RES_CTLR
    #include <linux/bit_spinlock.h>
    +#include <linux/hardirq.h>
    +
    /*
    * Page Cgroup can be considered as an extended mem_map.
    * A page_cgroup page is associated with every page descriptor. The
    @@ -119,12 +121,16 @@ static inline enum zone_type page_cgroup_zid(struct page_cgroup *pc)

    static inline void lock_page_cgroup(struct page_cgroup *pc)
    {
    + /* This routine is only deadlock safe from softirq or lower. */
    + VM_BUG_ON(in_irq());
    + local_bh_disable();
    bit_spin_lock(PCG_LOCK, &pc->flags);
    }

    static inline void unlock_page_cgroup(struct page_cgroup *pc)
    {
    bit_spin_unlock(PCG_LOCK, &pc->flags);
    + local_bh_enable();
    }

    #else /* CONFIG_CGROUP_MEM_RES_CTLR */
    --
    1.7.1


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