mm: add bdi_set_min_ratio_no_scale() function

This introduces bdi_set_min_ratio_no_scale(). It uses the max
granularity for the ratio. This function by the new sysfs knob
min_ratio_fine.

Link: https://lkml.kernel.org/r/20221119005215.3052436-19-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

@@ -109,6 +109,7 @@ 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);
+int bdi_set_min_ratio_no_scale(struct backing_dev_info *bdi, unsigned int min_ratio);
 int bdi_set_max_ratio_no_scale(struct backing_dev_info *bdi, unsigned int max_ratio);
 int bdi_set_min_bytes(struct backing_dev_info *bdi, u64 min_bytes);
 int bdi_set_max_bytes(struct backing_dev_info *bdi, u64 max_bytes);

mm/page-writeback.c

@@ -690,6 +690,8 @@ static int __bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ra
 	unsigned int delta;
 	int ret = 0;
 
+	if (min_ratio > 100 * BDI_RATIO_SCALE)
+		return -EINVAL;
 	min_ratio *= BDI_RATIO_SCALE;
 
 	spin_lock_bh(&bdi_lock);
@@ -734,6 +736,11 @@ static int __bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned int max_ra
 	return ret;
 }
 
+int bdi_set_min_ratio_no_scale(struct backing_dev_info *bdi, unsigned int min_ratio)
+{
+	return __bdi_set_min_ratio(bdi, min_ratio);
+}
+
 int bdi_set_max_ratio_no_scale(struct backing_dev_info *bdi, unsigned int max_ratio)
 {
 	return __bdi_set_max_ratio(bdi, max_ratio);