[RFC PATCH v3 05/14] mm: add bdi_get_max_bytes() function

[Stefan Roesch <shr@devkernel.io>]

This adds a function to return the specified value for max_bytes. It
converts the stored max_ratio of the bdi to the corresponding bytes
value. It introduces the bdi_get_bytes helper function to do the
conversion. This is an approximation as it is based on the value that is
returned by global_dirty_limits(), which can change. The helper function
will also be used by the min_bytes bdi knob.

Signed-off-by: Stefan Roesch <shr@devkernel.io>
---
 include/linux/backing-dev.h |  1 +
 mm/page-writeback.c         | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index 19fe0e605ed8..cb0e66564d4d 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -105,6 +105,7 @@ static inline unsigned long wb_stat_error(void)
 /* BDI ratio is expressed as part per 1000 for finer granularity. */
 #define BDI_RATIO_SCALE 10
 
+unsigned long long bdi_get_max_bytes(struct backing_dev_info *bdi);
 int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio);
 int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned int max_ratio);
 int bdi_set_strict_limit(struct backing_dev_info *bdi, unsigned int strict_limit);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 4d5383d4da45..0b9dcf5afda2 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -650,6 +650,18 @@ void wb_domain_exit(struct wb_domain *dom)
  */
 static unsigned int bdi_min_ratio;
 
+static unsigned long long bdi_get_bytes(unsigned int ratio)
+{
+	unsigned long background_thresh;
+	unsigned long dirty_thresh;
+	unsigned long long bytes;
+
+	global_dirty_limits(&background_thresh, &dirty_thresh);
+	bytes = (dirty_thresh * PAGE_SIZE * ratio) / BDI_RATIO_SCALE / 100;
+
+	return bytes;
+}
+
 int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
 {
 	unsigned int delta;
@@ -701,6 +713,12 @@ int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio)
 }
 EXPORT_SYMBOL(bdi_set_max_ratio);
 
+unsigned long long bdi_get_max_bytes(struct backing_dev_info *bdi)
+{
+	return bdi_get_bytes(bdi->max_ratio);
+}
+EXPORT_SYMBOL_GPL(bdi_get_max_bytes);
+
 int bdi_set_strict_limit(struct backing_dev_info *bdi, unsigned int strict_limit)
 {
 	if (strict_limit > 1)
-- 
2.30.2