[RFC PATCH v3 14/18] mm: Prepare balance_dirty_pages() for async buffered writes

[Stefan Roesch <shr@fb.com>]

From: Jan Kara <jack@suse.cz>

If balance_dirty_pages() gets called for async buffered write, we don't
want to wait. Instead we need to indicate to the caller that throttling
is needed so that it can stop writing and offload the rest of the write
to a context that can block.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Stefan Roesch <shr@fb.com>
---
 mm/page-writeback.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 89dcc7d8395a..fc3b79acd90b 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1545,8 +1545,8 @@ static inline void wb_dirty_limits(struct dirty_throttle_control *dtc)
  * If we're over `background_thresh' then the writeback threads are woken to
  * perform some writeout.
  */
-static void balance_dirty_pages(struct bdi_writeback *wb,
-				unsigned long pages_dirtied)
+static int balance_dirty_pages(struct bdi_writeback *wb,
+			       unsigned long pages_dirtied, bool nowait)
 {
 	struct dirty_throttle_control gdtc_stor = { GDTC_INIT(wb) };
 	struct dirty_throttle_control mdtc_stor = { MDTC_INIT(wb, &gdtc_stor) };
@@ -1566,6 +1566,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 	struct backing_dev_info *bdi = wb->bdi;
 	bool strictlimit = bdi->capabilities & BDI_CAP_STRICTLIMIT;
 	unsigned long start_time = jiffies;
+	int ret = 0;
 
 	for (;;) {
 		unsigned long now = jiffies;
@@ -1794,6 +1795,10 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 					  period,
 					  pause,
 					  start_time);
+		if (nowait) {
+			ret = -EAGAIN;
+			break;
+		}
 		__set_current_state(TASK_KILLABLE);
 		wb->dirty_sleep = now;
 		io_schedule_timeout(pause);
@@ -1825,6 +1830,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 		if (fatal_signal_pending(current))
 			break;
 	}
+	return ret;
 }
 
 static DEFINE_PER_CPU(int, bdp_ratelimits);
@@ -1906,7 +1912,7 @@ void balance_dirty_pages_ratelimited(struct address_space *mapping)
 	preempt_enable();
 
 	if (unlikely(current->nr_dirtied >= ratelimit))
-		balance_dirty_pages(wb, current->nr_dirtied);
+		balance_dirty_pages(wb, current->nr_dirtied, false);
 
 	wb_put(wb);
 }
-- 
2.30.2