mm: bdi: allow setting a maximum for the bdi dirty limit

Add "max_ratio" to /sys/class/bdi.  This indicates the maximum percentage of
the global dirty threshold allocated to this bdi.

[mszeredi@suse.cz]

 - fix parsing in max_ratio_store().
 - export bdi_set_max_ratio() to modules
 - limit bdi_dirty with bdi->max_ratio
 - document new sysfs attribute

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 56 insertions, 6 deletions

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 4967fb176e53..08361b6aad50 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -73,6 +73,24 @@ static ssize_t min_ratio_store(struct device *dev,
 }
 BDI_SHOW(min_ratio, bdi->min_ratio)
 
+static ssize_t max_ratio_store(struct device *dev,
+                struct device_attribute *attr, const char *buf, size_t count)
+{
+        struct backing_dev_info *bdi = dev_get_drvdata(dev);
+        char *end;
+        unsigned int ratio;
+        ssize_t ret = -EINVAL;
+
+        ratio = simple_strtoul(buf, &end, 10);
+        if (*buf && (end[0] == '\0' || (end[0] == '\n' && end[1] == '\0'))) {
+                ret = bdi_set_max_ratio(bdi, ratio);
+                if (!ret)
+                        ret = count;
+        }
+        return ret;
+}
+BDI_SHOW(max_ratio, bdi->max_ratio)
+
 #define __ATTR_RW(attr) __ATTR(attr, 0644, attr##_show, attr##_store)
 
 static struct device_attribute bdi_dev_attrs[] = {
@@ -82,6 +100,7 @@ static struct device_attribute bdi_dev_attrs[] = {
         __ATTR_RO(dirty_kb),
         __ATTR_RO(bdi_dirty_kb),
         __ATTR_RW(min_ratio),
+        __ATTR_RW(max_ratio),
         __ATTR_NULL,
 };
 
@@ -147,6 +166,8 @@ int bdi_init(struct backing_dev_info *bdi)
         bdi->dev = NULL;
 
         bdi->min_ratio = 0;
+        bdi->max_ratio = 100;
+        bdi->max_prop_frac = PROP_FRAC_BASE;
 
         for (i = 0; i < NR_BDI_STAT_ITEMS; i++) {
                 err = percpu_counter_init_irq(&bdi->bdi_stat[i], 0);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 4ac077f4269c..2a9942f5387c 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -164,7 +164,8 @@ int dirty_ratio_handler(struct ctl_table *table, int write,
  */
 static inline void __bdi_writeout_inc(struct backing_dev_info *bdi)
 {
-        __prop_inc_percpu(&vm_completions, &bdi->completions);
+        __prop_inc_percpu_max(&vm_completions, &bdi->completions,
+                              bdi->max_prop_frac);
 }
 
 static inline void task_dirty_inc(struct task_struct *tsk)
@@ -254,16 +255,42 @@ int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
         unsigned long flags;
 
         spin_lock_irqsave(&bdi_lock, flags);
-        min_ratio -= bdi->min_ratio;
-        if (bdi_min_ratio + min_ratio < 100) {
-                bdi_min_ratio += min_ratio;
-                bdi->min_ratio += min_ratio;
-        } else
+        if (min_ratio > bdi->max_ratio) {
                 ret = -EINVAL;
+        } else {
+                min_ratio -= bdi->min_ratio;
+                if (bdi_min_ratio + min_ratio < 100) {
+                        bdi_min_ratio += min_ratio;
+                        bdi->min_ratio += min_ratio;
+                } else {
+                        ret = -EINVAL;
+                }
+        }
+        spin_unlock_irqrestore(&bdi_lock, flags);
+
+        return ret;
+}
+
+int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio)
+{
+        unsigned long flags;
+        int ret = 0;
+
+        if (max_ratio > 100)
+                return -EINVAL;
+
+        spin_lock_irqsave(&bdi_lock, flags);
+        if (bdi->min_ratio > max_ratio) {
+                ret = -EINVAL;
+        } else {
+                bdi->max_ratio = max_ratio;
+                bdi->max_prop_frac = (PROP_FRAC_BASE * max_ratio) / 100;
+        }
         spin_unlock_irqrestore(&bdi_lock, flags);
 
         return ret;
 }
+EXPORT_SYMBOL(bdi_set_max_ratio);
 
 /*
  * Work out the current dirty-memory clamping and background writeout
@@ -365,6 +392,8 @@ get_dirty_limits(long *pbackground, long *pdirty, long *pbdi_dirty,
                 bdi_dirty *= numerator;
                 do_div(bdi_dirty, denominator);
                 bdi_dirty += (dirty * bdi->min_ratio) / 100;
+                if (bdi_dirty > (dirty * bdi->max_ratio) / 100)
+                        bdi_dirty = dirty * bdi->max_ratio / 100;
 
                 *pbdi_dirty = bdi_dirty;
                 clip_bdi_dirty_limit(bdi, dirty, pbdi_dirty);