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    Subject[PATCH 9/9] writeback: disassociate inodes from dying bdi_writebacks
    Date
    For the purpose of foreign inode detection, wb's (bdi_writeback's) are
    identified by the associated memcg ID. As we create a separate wb for
    each memcg, this is enough to identify the active wb's; however, when
    blkcg is enabled or disabled higher up in the hierarchy, the mapping
    between memcg and blkcg changes which in turn creates a new wb to
    service the new mapping. The old wb is unlinked from index and
    released after all references are drained. The foreign inode
    detection logic can't detect this condition because both the old and
    new wb's point to the same memcg and thus never decides to move inodes
    attached to the old wb to the new one.

    This patch adds logic to initiate switching immediately in
    wbc_attach_and_unlock_inode() if the associated wb is dying. We can
    make the usual foreign detection logic to distinguish the different
    wb's mapped to the memcg but the dying wb is never gonna be in active
    service again and there's no point in tracking the usage history and
    reaching the switch verdict after enough data points are collected.
    It's already known that the wb has to be switched.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    Cc: Jens Axboe <axboe@kernel.dk>
    Cc: Jan Kara <jack@suse.cz>
    Cc: Wu Fengguang <fengguang.wu@intel.com>
    Cc: Greg Thelen <gthelen@google.com>
    ---
    fs/fs-writeback.c | 7 +++++++
    include/linux/backing-dev-defs.h | 16 ++++++++++++++++
    2 files changed, 23 insertions(+)

    diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
    index 5eeb24a..f60de54 100644
    --- a/fs/fs-writeback.c
    +++ b/fs/fs-writeback.c
    @@ -525,6 +525,13 @@ void wbc_attach_and_unlock_inode(struct writeback_control *wbc,

    wb_get(wbc->wb);
    spin_unlock(&inode->i_lock);
    +
    + /*
    + * A dying wb indicates that the memcg-blkcg mapping has changed
    + * and a new wb is already serving the memcg. Switch immediately.
    + */
    + if (unlikely(wb_dying(wbc->wb)))
    + inode_switch_wbs(inode, wbc->wb_id);
    }

    /**
    diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
    index e047b49..a48d90e 100644
    --- a/include/linux/backing-dev-defs.h
    +++ b/include/linux/backing-dev-defs.h
    @@ -219,6 +219,17 @@ static inline void wb_put(struct bdi_writeback *wb)
    percpu_ref_put(&wb->refcnt);
    }

    +/**
    + * wb_dying - is a wb dying?
    + * @wb: bdi_writeback of interest
    + *
    + * Returns whether @wb is unlinked and being drained.
    + */
    +static inline bool wb_dying(struct bdi_writeback *wb)
    +{
    + return percpu_ref_is_dying(&wb->refcnt);
    +}
    +
    #else /* CONFIG_CGROUP_WRITEBACK */

    static inline bool wb_tryget(struct bdi_writeback *wb)
    @@ -234,6 +245,11 @@ static inline void wb_put(struct bdi_writeback *wb)
    {
    }

    +static inline bool wb_dying(struct bdi_writeback *wb)
    +{
    + return false;
    +}
    +
    #endif /* CONFIG_CGROUP_WRITEBACK */

    #endif /* __LINUX_BACKING_DEV_DEFS_H */
    --
    2.4.0


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