mm: add bdi_get_min_bytes() function

This adds a function to return the specified value for min_bytes. It
converts the stored min_ratio of the bdi to the corresponding bytes
value. This is an approximation as it is based on the value that is
returned by global_dirty_limits(), which can change. The returned
value can be different than the value when the min_bytes value was set.

Link: https://lkml.kernel.org/r/20221119005215.3052436-11-shr@devkernel.ioSigned-off-by: Stefan Roesch <shr@devkernel.io>
Cc: Chris Mason <clm@meta.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/backing-dev.h

@@ -105,6 +105,7 @@ static inline unsigned long wb_stat_error(void)
 /* BDI ratio is expressed as part per 1000000 for finer granularity. */
 #define BDI_RATIO_SCALE 10000
 
+u64 bdi_get_min_bytes(struct backing_dev_info *bdi);
 u64 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);

mm/page-writeback.c

@@ -740,6 +740,11 @@ int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned int max_ratio)
 }
 EXPORT_SYMBOL(bdi_set_max_ratio);
 
+u64 bdi_get_min_bytes(struct backing_dev_info *bdi)
+{
+	return bdi_get_bytes(bdi->min_ratio);
+}
+
 u64 bdi_get_max_bytes(struct backing_dev_info *bdi)
 {
 	return bdi_get_bytes(bdi->max_ratio);