From: Joe Thornber <ejt@redhat.com>

Be a bit cleverer about calculating the threshold at which we notify
userspace that we are running out of space for thin metadata.

When we get low on free metadata blocks we trigger a dm event, allowing
userland to resize the metadata device.

Previously this threshold was a percentage of the metadata device.

Now it's the minimum of 4M or a percentage.  Should stop us triggering
too early for very large metadata device.

Signed-off-by: Joe Thornber <ejt@redhat.com>

Index: linux/drivers/md/dm-thin.c
===================================================================
--- linux.orig/drivers/md/dm-thin.c
+++ linux/drivers/md/dm-thin.c
@@ -42,14 +42,6 @@ DECLARE_DM_KCOPYD_THROTTLE_WITH_MODULE_P
 #define MAX_DEV_ID ((1 << 24) - 1)
 
 /*
- * Metadata low water mark.
- *
- * If the free blocks in the metadata device pass this threshold an event
- * will be generated.
- */
-#define METADATA_LOW_WATER_MARK 64
-
-/*
  * How do we handle breaking sharing of data blocks?
  * =================================================
  *
@@ -1289,6 +1281,10 @@ static void process_bio_fail(struct thin
 	bio_io_error(bio);
 }
 
+/*
+ * FIXME: should we also commit due to size of transaction, measured in
+ * metadata blocks?
+ */
 static int need_commit_due_to_time(struct pool *pool)
 {
 	return jiffies < pool->last_commit_jiffies ||
@@ -1951,6 +1947,23 @@ static dm_block_t get_metadata_dev_size_
 }
 
 /*
+ * When a metadata threshold is crossed a dm event is triggered, and
+ * userland should respond by growing the metadata device.  We could let
+ * userland set the threshold, like we do with the data threshold, but I'm
+ * not sure they know enough to do this well.
+ */
+static dm_block_t calc_metadata_threshold(struct pool_c *pt)
+{
+	/*
+	 * 4M is ample for all ops with the possible exception of thin
+	 * device deletion which is harmless if it fails (just retry the
+	 * delete after you've grown the device).
+	 */
+	dm_block_t quarter = get_metadata_dev_size_in_blocks(pt->metadata_dev->bdev) / 4;
+	return min((dm_block_t)1024ULL /* 4M */, quarter);
+}
+
+/*
  * thin-pool <metadata dev> <data dev>
  *	     <data block size (sectors)>
  *	     <low water mark (blocks)>
@@ -2084,7 +2097,7 @@ static int pool_ctr(struct dm_target *ti
 	ti->private = pt;
 
 	r = dm_pool_register_metadata_threshold(pt->pool->pmd,
-						METADATA_LOW_WATER_MARK,
+						calc_metadata_threshold(pt),
 						metadata_low_callback,
 						pool);
 	if (r)