xen: Add selfballoning memory reservation tunable.

Currently, the memory target in the Xen selfballooning driver is mainly
driven by the value of "Committed_AS".  However, there are cases in
which it is desirable to assign additional memory to be available for
the kernel, e.g. for local caches (which are not covered by cleancache),
e.g. dcache and inode caches.

This adds an additional tunable in the selfballooning driver (accessible
via sysfs) which allows the user to specify an additional constant
amount of memory to be reserved by the selfballoning driver for the
local domain.
Signed-off-by: Jana Saout <jana@saout.de>
Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/xen-selfballoon.c

@@ -105,6 +105,12 @@ static unsigned int selfballoon_interval __read_mostly = 5;
  */
 static unsigned int selfballoon_min_usable_mb;
 
+/*
+ * Amount of RAM in MB to add to the target number of pages.
+ * Can be used to reserve some more room for caches and the like.
+ */
+static unsigned int selfballoon_reserved_mb;
+
 static void selfballoon_process(struct work_struct *work);
 static DECLARE_DELAYED_WORK(selfballoon_worker, selfballoon_process);
 
@@ -217,7 +223,8 @@ static void selfballoon_process(struct work_struct *work)
 		cur_pages = totalram_pages;
 		tgt_pages = cur_pages; /* default is no change */
 		goal_pages = percpu_counter_read_positive(&vm_committed_as) +
-				totalreserve_pages;
+				totalreserve_pages +
+				MB2PAGES(selfballoon_reserved_mb);
 #ifdef CONFIG_FRONTSWAP
 		/* allow space for frontswap pages to be repatriated */
 		if (frontswap_selfshrinking && frontswap_enabled)
@@ -397,6 +404,30 @@ static DEVICE_ATTR(selfballoon_min_usable_mb, S_IRUGO | S_IWUSR,
 		   show_selfballoon_min_usable_mb,
 		   store_selfballoon_min_usable_mb);
 
+SELFBALLOON_SHOW(selfballoon_reserved_mb, "%d\n",
+				selfballoon_reserved_mb);
+
+static ssize_t store_selfballoon_reserved_mb(struct device *dev,
+					     struct device_attribute *attr,
+					     const char *buf,
+					     size_t count)
+{
+	unsigned long val;
+	int err;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+	err = strict_strtoul(buf, 10, &val);
+	if (err || val == 0)
+		return -EINVAL;
+	selfballoon_reserved_mb = val;
+	return count;
+}
+
+static DEVICE_ATTR(selfballoon_reserved_mb, S_IRUGO | S_IWUSR,
+		   show_selfballoon_reserved_mb,
+		   store_selfballoon_reserved_mb);
+
 
 #ifdef CONFIG_FRONTSWAP
 SELFBALLOON_SHOW(frontswap_selfshrinking, "%d\n", frontswap_selfshrinking);
@@ -480,6 +511,7 @@ static struct attribute *selfballoon_attrs[] = {
 	&dev_attr_selfballoon_downhysteresis.attr,
 	&dev_attr_selfballoon_uphysteresis.attr,
 	&dev_attr_selfballoon_min_usable_mb.attr,
+	&dev_attr_selfballoon_reserved_mb.attr,
 #ifdef CONFIG_FRONTSWAP
 	&dev_attr_frontswap_selfshrinking.attr,
 	&dev_attr_frontswap_hysteresis.attr,