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-rw-r--r--mm/page-writeback.c32
1 files changed, 28 insertions, 4 deletions
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 71252486bc6..50f08241f98 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -411,8 +411,13 @@ void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
*
* Returns @bdi's dirty limit in pages. The term "dirty" in the context of
* dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages.
- * And the "limit" in the name is not seriously taken as hard limit in
- * balance_dirty_pages().
+ *
+ * Note that balance_dirty_pages() will only seriously take it as a hard limit
+ * when sleeping max_pause per page is not enough to keep the dirty pages under
+ * control. For example, when the device is completely stalled due to some error
+ * conditions, or when there are 1000 dd tasks writing to a slow 10MB/s USB key.
+ * In the other normal situations, it acts more gently by throttling the tasks
+ * more (rather than completely block them) when the bdi dirty pages go high.
*
* It allocates high/low dirty limits to fast/slow devices, in order to prevent
* - starving fast devices
@@ -594,6 +599,13 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
*/
if (unlikely(bdi_thresh > thresh))
bdi_thresh = thresh;
+ /*
+ * It's very possible that bdi_thresh is close to 0 not because the
+ * device is slow, but that it has remained inactive for long time.
+ * Honour such devices a reasonable good (hopefully IO efficient)
+ * threshold, so that the occasional writes won't be blocked and active
+ * writes can rampup the threshold quickly.
+ */
bdi_thresh = max(bdi_thresh, (limit - dirty) / 8);
/*
* scale global setpoint to bdi's:
@@ -977,8 +989,7 @@ static unsigned long bdi_max_pause(struct backing_dev_info *bdi,
*
* 8 serves as the safety ratio.
*/
- if (bdi_dirty)
- t = min(t, bdi_dirty * HZ / (8 * bw + 1));
+ t = min(t, bdi_dirty * HZ / (8 * bw + 1));
/*
* The pause time will be settled within range (max_pause/4, max_pause).
@@ -1136,6 +1147,19 @@ pause:
if (task_ratelimit)
break;
+ /*
+ * In the case of an unresponding NFS server and the NFS dirty
+ * pages exceeds dirty_thresh, give the other good bdi's a pipe
+ * to go through, so that tasks on them still remain responsive.
+ *
+ * In theory 1 page is enough to keep the comsumer-producer
+ * pipe going: the flusher cleans 1 page => the task dirties 1
+ * more page. However bdi_dirty has accounting errors. So use
+ * the larger and more IO friendly bdi_stat_error.
+ */
+ if (bdi_dirty <= bdi_stat_error(bdi))
+ break;
+
if (fatal_signal_pending(current))
break;
}