mm: add knob /sys/class/bdi/<bdi>/strict_limit

Add a new knob to /sys/class/bdi/<bdi>/strict_limit. This new knob
allows to set/unset the flag BDI_CAP_STRICTLIMIT in the bdi
capabilities.

Link: https://lkml.kernel.org/r/20221119005215.3052436-3-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>

mm/backing-dev.c

@@ -209,11 +209,40 @@ static ssize_t stable_pages_required_show(struct device *dev,
 }
 static DEVICE_ATTR_RO(stable_pages_required);
 
+static ssize_t strict_limit_store(struct device *dev,
+		struct device_attribute *attr, const char *buf, size_t count)
+{
+	struct backing_dev_info *bdi = dev_get_drvdata(dev);
+	unsigned int strict_limit;
+	ssize_t ret;
+
+	ret = kstrtouint(buf, 10, &strict_limit);
+	if (ret < 0)
+		return ret;
+
+	ret = bdi_set_strict_limit(bdi, strict_limit);
+	if (!ret)
+		ret = count;
+
+	return ret;
+}
+
+static ssize_t strict_limit_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	struct backing_dev_info *bdi = dev_get_drvdata(dev);
+
+	return sysfs_emit(buf, "%d\n",
+			!!(bdi->capabilities & BDI_CAP_STRICTLIMIT));
+}
+static DEVICE_ATTR_RW(strict_limit);
+
 static struct attribute *bdi_dev_attrs[] = {
 	&dev_attr_read_ahead_kb.attr,
 	&dev_attr_min_ratio.attr,
 	&dev_attr_max_ratio.attr,
 	&dev_attr_stable_pages_required.attr,
+	&dev_attr_strict_limit.attr,
 	NULL,
 };
 ATTRIBUTE_GROUPS(bdi_dev);