mm: add strictlimit knob

The "strictlimit" feature was introduced to enforce per-bdi dirty limits
for FUSE which sets bdi max_ratio to 1% by default:

http://article.gmane.org/gmane.linux.kernel.mm/105809

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.

Signed-off-by: Maxim Patlasov <MPatlasov@parallels.com>
Cc: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: "Artem S. Tashkinov" <t.artem@lycos.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Jan Kara <jack@suse.cz>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/Documentation/ABI/testing/sysfs-class-bdi b/Documentation/ABI/testing/sysfs-class-bdi
index d773d5697cf588349b74fd7d19e63983472b1632..3187a18af6da5bbbcc9ee2a0592e27f62ab1983c 100644 (file)
--- a/Documentation/ABI/testing/sysfs-class-bdi
+++ b/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 --git a/mm/backing-dev.c b/mm/backing-dev.c
index 1706cbbdf5f0381aaf81f21f6bc47b1133e2746b..382f8ece1cb27db25c0fbc6c856bbc5ac9fc7d57 100644 (file)
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -234,11 +234,46 @@ static ssize_t stable_pages_required_show(struct device *dev,
 }
 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);