However the feature can be useful for other relatively slow or untrusted
BDIs like USB flash drives and DVD+RW.  The patch adds a knob to enable
the feature:

echo 1 > /sys/class/bdi/X:Y/strictlimit

Being enabled, the feature enforces bdi max_ratio limit even if global
(10%) dirty limit is not reached.  Of course, the effect is not visible
until /sys/class/bdi/X:Y/max_ratio is decreased to some reasonable value.

Jan said:

: In principle I have nothing against this and the usecase sounds reasonable
: (in fact I believe the lack of a feature like this is one of reasons why
: desktop automounters usually mount USB devices with 'sync' mount option).
: So feel free to add:

Signed-off-by: Maxim Patlasov <redacted>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: "Artem S. Tashkinov" <redacted>
Cc: Mel Gorman <redacted>
Cc: Jan Kara <jack@suse.cz>
Cc: Wu Fengguang <redacted>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

Documentation/ABI/testing/sysfs-class-bdi |    8 ++++
mm/backing-dev.c                          |   35 ++++++++++++++++++++
2 files changed, 43 insertions(+)

diff -puN Documentation/ABI/testing/sysfs-class-bdi~mm-add-strictlimit-knob-v2 Documentation/ABI/testing/sysfs-class-bdi

--- a/Documentation/ABI/testing/sysfs-class-bdi~mm-add-strictlimit-knob-v2
+++ a/Documentation/ABI/testing/sysfs-class-bdi

@@ -53,3 +53,11 @@ stable_pages_required (read-only)

	If set, the backing device requires that all pages comprising a write
	request must not be changed until writeout is complete.
+
+strictlimit (read-write)
+
+	Forces per-BDI checks for the share of given device in the write-back
+	cache even before the global background dirty limit is reached. This
+	is useful in situations where the global limit is much higher than
+	affordable for given relatively slow (or untrusted) device. Turning
+	strictlimit on has no visible effect if max_ratio is equal to 100%.
diff -puN mm/backing-dev.c~mm-add-strictlimit-knob-v2 mm/backing-dev.c

--- a/mm/backing-dev.c~mm-add-strictlimit-knob-v2
+++ a/mm/backing-dev.c

@@ -231,11 +231,46 @@ static ssize_t stable_pages_required_sho

}
static DEVICE_ATTR_RO(stable_pages_required);

+static ssize_t strictlimit_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 val;
+	ssize_t ret;
+
+	ret = kstrtouint(buf, 10, &val);
+	if (ret < 0)
+		return ret;
+
+	switch (val) {
+	case 0:
+		bdi->capabilities &= ~BDI_CAP_STRICTLIMIT;
+		break;
+	case 1:
+		bdi->capabilities |= BDI_CAP_STRICTLIMIT;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	return count;
+}
+static ssize_t strictlimit_show(struct device *dev,
+		struct device_attribute *attr, char *page)
+{
+	struct backing_dev_info *bdi = dev_get_drvdata(dev);
+
+	return snprintf(page, PAGE_SIZE-1, "%d\n",
+			!!(bdi->capabilities & BDI_CAP_STRICTLIMIT));
+}
+static DEVICE_ATTR_RW(strictlimit);
+
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_strictlimit.attr,
	NULL,
};
ATTRIBUTE_GROUPS(bdi_dev);
_