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, 3 insertions, 2 deletions

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index c24eca71e80c..72a5d647a5f2 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -124,8 +124,9 @@ struct ctl_table;
 int dirty_writeback_centisecs_handler(struct ctl_table *, int,
                                       void __user *, size_t *, loff_t *);
 
-void get_dirty_limits(unsigned long *pbackground, unsigned long *pdirty,
-                      unsigned long *pbdi_dirty, struct backing_dev_info *bdi);
+void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty);
+unsigned long bdi_dirty_limit(struct backing_dev_info *bdi,
+                               unsigned long dirty);
 
 void page_writeback_init(void);
 void balance_dirty_pages_ratelimited_nr(struct address_space *mapping,