[RFC PATCH v2 13/16] mm: add balance_dirty_pages_ratelimited_flags() function

[Stefan Roesch <shr@fb.com>]

This adds the function balance_dirty_pages_ratelimited_flags(). It adds
the parameter is_async to balance_dirty_pages_ratelimited(). In case
this is an async write, it will call _balance_diirty_pages() to
determine if write throttling needs to be enabled. If write throttling
is enabled, it retuns -EAGAIN, so the write request can be punted to
the io-uring worker.

The new function is changed to return the sleep time, so callers can
observe if the write has been punted.

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       | 48 ++++++++++++++++++++++++++-------------
 2 files changed, 33 insertions(+), 16 deletions(-)

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index fec248ab1fec..d589804bb3be 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_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 cbb74c0666c6..78f1326f3f20 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1877,28 +1877,17 @@ 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)
+int 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);
 	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);
@@ -1937,10 +1926,37 @@ void balance_dirty_pages_ratelimited(struct address_space *mapping)
 	}
 	preempt_enable();
 
-	if (unlikely(current->nr_dirtied >= ratelimit))
-		balance_dirty_pages(wb, current->nr_dirtied);
+	if (unlikely(current->nr_dirtied >= ratelimit)) {
+		if (is_async) {
+			struct bdp_ctx ctx = { BDP_CTX_INIT(ctx, wb) };
+
+			ret = _balance_dirty_pages(wb, current->nr_dirtied, &ctx);
+			if (ret)
+				ret = -EAGAIN;
+		} else {
+			balance_dirty_pages(wb, current->nr_dirtied);
+		}
+	}
 
 	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);
 
-- 
2.30.2