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>

1 files changed, 35 insertions, 6 deletions

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);