mm: add bdi_set_min_bytes() function

This introduces the bdi_set_min_bytes() function. The min_bytes function
does not store the min_bytes value. Instead it converts the min_bytes
value into the corresponding ratio value.

Link: https://lkml.kernel.org/r/20221119005215.3052436-13-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_bytes(struct backing_dev_info *bdi, u64 min_bytes);
 int bdi_set_max_bytes(struct backing_dev_info *bdi, u64 max_bytes);
 int bdi_set_strict_limit(struct backing_dev_info *bdi, unsigned int strict_limit);
 

mm/page-writeback.c

@@ -750,6 +750,20 @@ u64 bdi_get_min_bytes(struct backing_dev_info *bdi)
 	return bdi_get_bytes(bdi->min_ratio);
 }
 
+int bdi_set_min_bytes(struct backing_dev_info *bdi, u64 min_bytes)
+{
+	int ret;
+	unsigned long pages = min_bytes >> PAGE_SHIFT;
+	unsigned long min_ratio;
+
+	ret = bdi_check_pages_limit(pages);
+	if (ret)
+		return ret;
+
+	min_ratio = bdi_ratio_from_pages(pages);
+	return __bdi_set_min_ratio(bdi, min_ratio);
+}
+
 u64 bdi_get_max_bytes(struct backing_dev_info *bdi)
 {
 	return bdi_get_bytes(bdi->max_ratio);