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

bdi has two existing knobs to limit the amount of dirty memory:
min_ratio and max_ratio. However the granularity of the knobs is limited
and often it is more convenient to specify limits in terms of bytes.
This change adds the min_bytes knob.

It does not store the min_bytes value, instead it converts the max_bytes
value to a ratio. The value is therefore more an approximation than an
absolute value.

It also maintains the sum over all the bdi min_ratio values stored in
the variable bdi_min_ratio.

Link: https://lkml.kernel.org/r/20221119005215.3052436-14-shr@devkernel.io
Signed-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>

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 95d3229fc81f002005c431a5a30ca27aa34c9f6f..3fab79061adebf7d3a2effe56adcd526ccfafce8 100644 (file)
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -199,6 +199,34 @@ static ssize_t max_ratio_store(struct device *dev,
 }
 BDI_SHOW(max_ratio, bdi->max_ratio / BDI_RATIO_SCALE)
 
+static ssize_t min_bytes_show(struct device *dev,
+                             struct device_attribute *attr,
+                             char *buf)
+{
+       struct backing_dev_info *bdi = dev_get_drvdata(dev);
+
+       return sysfs_emit(buf, "%llu\n", bdi_get_min_bytes(bdi));
+}
+
+static ssize_t min_bytes_store(struct device *dev,
+               struct device_attribute *attr, const char *buf, size_t count)
+{
+       struct backing_dev_info *bdi = dev_get_drvdata(dev);
+       u64 bytes;
+       ssize_t ret;
+
+       ret = kstrtoull(buf, 10, &bytes);
+       if (ret < 0)
+               return ret;
+
+       ret = bdi_set_min_bytes(bdi, bytes);
+       if (!ret)
+               ret = count;
+
+       return ret;
+}
+DEVICE_ATTR_RW(min_bytes);
+
 static ssize_t max_bytes_show(struct device *dev,
                              struct device_attribute *attr,
                              char *buf)
@@ -269,6 +297,7 @@ static struct attribute *bdi_dev_attrs[] = {
        &dev_attr_read_ahead_kb.attr,
        &dev_attr_min_ratio.attr,
        &dev_attr_max_ratio.attr,
+       &dev_attr_min_bytes.attr,
        &dev_attr_max_bytes.attr,
        &dev_attr_stable_pages_required.attr,
        &dev_attr_strict_limit.attr,