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

Under normal circumstances each device is given a part of the total write-back
cache that relates to its current avg writeout speed in relation to the other
devices.

min_ratio - allows one to assign a minimum portion of the write-back cache to
a particular device.  This is useful in situations where you might want to
provide a minimum QoS.  (One request for this feature came from flash based
storage people who wanted to avoid writing out at all costs - they of course
needed some pdflush hacks as well)

max_ratio - allows one to assign a maximum portion of the dirty limit to a
particular device.  This is useful in situations where you want to avoid one
device taking all or most of the write-back cache.  Eg.  an NFS mount that is
prone to get stuck, or a FUSE mount which you don't trust to play fair.

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

[mszeredi@suse.cz]

 - fix parsing in min_ratio_store()
 - 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, 4 insertions, 0 deletions

diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index 6d513666d45c..9a8965518d1d 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -51,6 +51,8 @@ struct backing_dev_info {
         struct prop_local_percpu completions;
         int dirty_exceeded;
 
+        unsigned int min_ratio;
+
         struct device *dev;
 };
 
@@ -137,6 +139,8 @@ static inline unsigned long bdi_stat_error(struct backing_dev_info *bdi)
 #endif
 }
 
+int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio);
+
 /*
  * Flags in backing_dev_info::capability
  * - The first two flags control whether dirty pages will contribute to the