[RFC PATCH v3 15/18] mm: Add balance_dirty_pages_ratelimited_async() function

[Stefan Roesch <shr@fb.com>]

This adds the helper function balance_dirty_pages_ratelimited_flags().
It adds the parameter no_wait to balance_dirty_pages_ratelimited().
For async buffered writes no_wait will be true.
A new function called balance_dirty_pages_ratelimited_async() is
introduced that calls balance_dirty_pages_ratelimited_flags with no_wait
set to true.
If write throttling is enabled, it retuns -EAGAIN, so the write request
can be punted to the io-uring worker.

For non-async writes the current behavior is maintained.

Signed-off-by: Stefan Roesch <shr@fb.com>
---
 include/linux/writeback.h |  1 +
 mm/page-writeback.c       | 60 +++++++++++++++++++++++++++++----------
 2 files changed, 46 insertions(+), 15 deletions(-)

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index fec248ab1fec..15eb0242d3ef 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);
+int  balance_dirty_pages_ratelimited_async(struct address_space *mapping);
 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 fc3b79acd90b..d6a67fc07c55 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1851,28 +1851,18 @@ 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)
+static int balance_dirty_pages_ratelimited_flags(struct address_space *mapping,
+						bool no_wait)
 {
 	struct inode *inode = mapping->host;
 	struct backing_dev_info *bdi = inode_to_bdi(inode);
 	struct bdi_writeback *wb = NULL;
 	int ratelimit;
+	int ret = 0;
 	int *p;
 
 	if (!(bdi->capabilities & BDI_CAP_WRITEBACK))
-		return;
+		return ret;
 
 	if (inode_cgwb_enabled(inode))
 		wb = wb_get_create_current(bdi, GFP_KERNEL);
@@ -1912,12 +1902,52 @@ void balance_dirty_pages_ratelimited(struct address_space *mapping)
 	preempt_enable();
 
 	if (unlikely(current->nr_dirtied >= ratelimit))
-		balance_dirty_pages(wb, current->nr_dirtied, false);
+		balance_dirty_pages(wb, current->nr_dirtied, no_wait);
 
 	wb_put(wb);
+	return ret;
+}
+
+/**
+ * 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);
 
+/**
+ * balance_dirty_pages_ratelimited_async - 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.
+ *
+ * This is the async version of the API. It only checks if it is required to
+ * balance dirty pages. In case it needs to balance dirty pages, it returns
+ * -EAGAIN.
+ */
+int  balance_dirty_pages_ratelimited_async(struct address_space *mapping)
+{
+	return balance_dirty_pages_ratelimited_flags(mapping, true);
+}
+EXPORT_SYMBOL(balance_dirty_pages_ratelimited_async);
+
 /**
  * wb_over_bg_thresh - does @wb need to be written back?
  * @wb: bdi_writeback of interest
-- 
2.30.2