writeback: avoid unnecessary calculation of bdi dirty thresholds

Split get_dirty_limits() into global_dirty_limits()+bdi_dirty_limit(), so
that the latter can be avoided when under global dirty background
threshold (which is the normal state for most systems).

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 08d357522e78..eaa4a5bbe063 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -81,7 +81,8 @@ static int bdi_debug_stats_show(struct seq_file *m, void *v)
                 nr_more_io++;
         spin_unlock(&inode_lock);
 
-        get_dirty_limits(&background_thresh, &dirty_thresh, &bdi_thresh, bdi);
+        global_dirty_limits(&background_thresh, &dirty_thresh);
+        bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
 
 #define K(x) ((x) << (PAGE_SHIFT - 10))
         seq_printf(m,