[RFC PATCH v1 15/18] mm: support write throttling for async buffered writes

[Stefan Roesch <shr@fb.com>]

This change adds support for async write throttling in the function
balance_dirty_pages(). So far if throttling was required, the code was
waiting synchronously as long as the writes were throttled. This change
introduces asynchronous throttling. Instead of waiting in the function
balance_dirty_pages(), the timeout is set in the task_struct field
bdp_pause. Once the timeout has expired, the writes are no longer
throttled.

- Add a new parameter to the balance_dirty_pages() function
  - This allows the caller to pass in the nowait flag
  - When the nowait flag is specified, the code does not wait in
    balance_dirty_pages(), but instead stores the wait expiration in the
    new task_struct field bdp_pause.

- The function balance_dirty_pages_ratelimited() resets the new values
  in the task_struct, once the timeout has expired

This change is required to support write throttling for the async
buffered writes. While the writes are throttled, io_uring still can make
progress with processing other requests.

Signed-off-by: Stefan Roesch <shr@fb.com>
---
 include/linux/writeback.h |  1 +
 mm/page-writeback.c       | 54 ++++++++++++++++++++++++++++-----------
 2 files changed, 40 insertions(+), 15 deletions(-)

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index fec248ab1fec..48176a8047db 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -373,6 +373,7 @@ unsigned long wb_calc_thresh(struct bdi_writeback *wb, unsigned long thresh);
 
 void wb_update_bandwidth(struct bdi_writeback *wb);
 void balance_dirty_pages_ratelimited(struct address_space *mapping);
+void  balance_dirty_pages_ratelimited_flags(struct address_space *mapping, bool is_async);
 bool wb_over_bg_thresh(struct bdi_writeback *wb);
 
 typedef int (*writepage_t)(struct page *page, struct writeback_control *wbc,
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 7e2da284e427..a62aa8a4c2f2 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1546,7 +1546,7 @@ static inline void wb_dirty_limits(struct dirty_throttle_control *dtc)
  * perform some writeout.
  */
 static void balance_dirty_pages(struct bdi_writeback *wb,
-				unsigned long pages_dirtied)
+				unsigned long pages_dirtied, bool is_async)
 {
 	struct dirty_throttle_control gdtc_stor = { GDTC_INIT(wb) };
 	struct dirty_throttle_control mdtc_stor = { MDTC_INIT(wb, &gdtc_stor) };
@@ -1780,6 +1780,14 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 					  period,
 					  pause,
 					  start_time);
+		if (is_async) {
+			if (current->bdp_nr_dirtied_pause == -1) {
+				current->bdp_pause = now + pause;
+				current->bdp_nr_dirtied_pause = nr_dirtied_pause;
+			}
+			break;
+		}
+
 		__set_current_state(TASK_KILLABLE);
 		wb->dirty_sleep = now;
 		io_schedule_timeout(pause);
@@ -1787,6 +1795,8 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 		current->dirty_paused_when = now + pause;
 		current->nr_dirtied = 0;
 		current->nr_dirtied_pause = nr_dirtied_pause;
+		current->bdp_nr_dirtied_pause = -1;
+		current->bdp_pause = 0;
 
 		/*
 		 * This is typically equal to (dirty < thresh) and can also
@@ -1851,19 +1861,7 @@ static DEFINE_PER_CPU(int, bdp_ratelimits);
  */
 DEFINE_PER_CPU(int, dirty_throttle_leaks) = 0;
 
-/**
- * balance_dirty_pages_ratelimited - balance dirty memory state
- * @mapping: address_space which was dirtied
- *
- * Processes which are dirtying memory should call in here once for each page
- * which was newly dirtied.  The function will periodically check the system's
- * dirty state and will initiate writeback if needed.
- *
- * Once we're over the dirty memory limit we decrease the ratelimiting
- * by a lot, to prevent individual processes from overshooting the limit
- * by (ratelimit_pages) each.
- */
-void balance_dirty_pages_ratelimited(struct address_space *mapping)
+void balance_dirty_pages_ratelimited_flags(struct address_space *mapping, bool is_async)
 {
 	struct inode *inode = mapping->host;
 	struct backing_dev_info *bdi = inode_to_bdi(inode);
@@ -1874,6 +1872,15 @@ void balance_dirty_pages_ratelimited(struct address_space *mapping)
 	if (!(bdi->capabilities & BDI_CAP_WRITEBACK))
 		return;
 
+	if (current->bdp_nr_dirtied_pause != -1 && time_after(jiffies, current->bdp_pause)) {
+		current->dirty_paused_when = current->bdp_pause;
+		current->nr_dirtied = 0;
+		current->nr_dirtied_pause = current->bdp_nr_dirtied_pause;
+
+		current->bdp_nr_dirtied_pause = -1;
+		current->bdp_pause = 0;
+	}
+
 	if (inode_cgwb_enabled(inode))
 		wb = wb_get_create_current(bdi, GFP_KERNEL);
 	if (!wb)
@@ -1912,10 +1919,27 @@ 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, is_async);
 
 	wb_put(wb);
 }
+
+/**
+ * balance_dirty_pages_ratelimited - balance dirty memory state
+ * @mapping: address_space which was dirtied
+ *
+ * Processes which are dirtying memory should call in here once for each page
+ * which was newly dirtied.  The function will periodically check the system's
+ * dirty state and will initiate writeback if needed.
+ *
+ * Once we're over the dirty memory limit we decrease the ratelimiting
+ * by a lot, to prevent individual processes from overshooting the limit
+ * by (ratelimit_pages) each.
+ */
+void balance_dirty_pages_ratelimited(struct address_space *mapping)
+{
+	balance_dirty_pages_ratelimited_flags(mapping, false);
+}
 EXPORT_SYMBOL(balance_dirty_pages_ratelimited);
 
 /**
-- 
2.30.2