writeback: remove the internal 5% low bound on dirty_ratio

The dirty_ratio was silently limited in global_dirty_limits() to >= 5%.
This is not a user expected behavior.  And it's inconsistent with
calc_period_shift(), which uses the plain vm_dirty_ratio value.

Let's remove the internal bound.

At the same time, fix balance_dirty_pages() to work with the
dirty_thresh=0 case.  This allows applications to proceed when
dirty+writeback pages are all cleaned.

And ">" fits with the name "exceeded" better than ">=" does.  Neil thinks
it is an aesthetic improvement as well as a functional one :)

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Cc: Jan Kara <jack@suse.cz>
Proposed-by: Con Kolivas <kernel@kolivas.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reviewed-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Michael Rubin <mrubin@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 6 insertions, 12 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index ab38fef1c9a1..97d2951bd4d1 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -582,7 +582,7 @@ static inline bool over_bground_thresh(void)
         global_dirty_limits(&background_thresh, &dirty_thresh);
 
         return (global_page_state(NR_FILE_DIRTY) +
-                global_page_state(NR_UNSTABLE_NFS) >= background_thresh);
+                global_page_state(NR_UNSTABLE_NFS) > background_thresh);
 }
 
 /*
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 4dd91f7fd39f..b840afa89761 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -415,14 +415,8 @@ void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
 
         if (vm_dirty_bytes)
                 dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
-        else {
-                int dirty_ratio;
-
-                dirty_ratio = vm_dirty_ratio;
-                if (dirty_ratio < 5)
-                        dirty_ratio = 5;
-                dirty = (dirty_ratio * available_memory) / 100;
-        }
+        else
+                dirty = (vm_dirty_ratio * available_memory) / 100;
 
         if (dirty_background_bytes)
                 background = DIV_ROUND_UP(dirty_background_bytes, PAGE_SIZE);
@@ -510,7 +504,7 @@ static void balance_dirty_pages(struct address_space *mapping,
                  * catch-up. This avoids (excessively) small writeouts
                  * when the bdi limits are ramping up.
                  */
-                if (nr_reclaimable + nr_writeback <
+                if (nr_reclaimable + nr_writeback <=
                                 (background_thresh + dirty_thresh) / 2)
                         break;
 
@@ -542,8 +536,8 @@ static void balance_dirty_pages(struct address_space *mapping,
                  * the last resort safeguard.
                  */
                 dirty_exceeded =
-                        (bdi_nr_reclaimable + bdi_nr_writeback >= bdi_thresh)
-                        || (nr_reclaimable + nr_writeback >= dirty_thresh);
+                        (bdi_nr_reclaimable + bdi_nr_writeback > bdi_thresh)
+                        || (nr_reclaimable + nr_writeback > dirty_thresh);
 
                 if (!dirty_exceeded)
                         break;