UBIFS: document dark_wm and dead_wm better

Just add more commentaries. Also some commentary fixes for
lprops flags.
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

fs/ubifs/gc.c

@@ -31,6 +31,26 @@
  * to be reused. Garbage collection will cause the number of dirty index nodes
  * to grow, however sufficient space is reserved for the index to ensure the
  * commit will never run out of space.
+ *
+ * Notes about dead watermark. At current UBIFS implementation we assume that
+ * LEBs which have less than @c->dead_wm bytes of free + dirty space are full
+ * and not worth garbage-collecting. The dead watermark is one min. I/O unit
+ * size, or min. UBIFS node size, depending on what is greater. Indeed, UBIFS
+ * Garbage Collector has to synchronize the GC head's write buffer before
+ * returning, so this is about wasting one min. I/O unit. However, UBIFS GC can
+ * actually reclaim even very small pieces of dirty space by garbage collecting
+ * enough dirty LEBs, but we do not bother doing this at this implementation.
+ *
+ * Notes about dark watermark. The results of GC work depends on how big are
+ * the UBIFS nodes GC deals with. Large nodes make GC waste more space. Indeed,
+ * if GC move data from LEB A to LEB B and nodes in LEB A are large, GC would
+ * have to waste large pieces of free space at the end of LEB B, because nodes
+ * from LEB A would not fit. And the worst situation is when all nodes are of
+ * maximum size. So dark watermark is the amount of free + dirty space in LEB
+ * which are guaranteed to be reclaimable. If LEB has less space, the GC migh
+ * be unable to reclaim it. So, LEBs with free + dirty greater than dark
+ * watermark are "good" LEBs from GC's point of few. The other LEBs are not so
+ * good, and GC takes extra care when moving them.
  */
 
 #include <linux/pagemap.h>

fs/ubifs/super.c

@@ -573,15 +573,8 @@ static int init_constants_early(struct ubifs_info *c)
 	c->ranges[UBIFS_IDX_NODE].max_len = INT_MAX;
 
 	/*
-	 * Initialize dead and dark LEB space watermarks.
-	 *
-	 * Dead space is the space which cannot be used. Its watermark is
-	 * equivalent to min. I/O unit or minimum node size if it is greater
-	 * then min. I/O unit.
-	 *
-	 * Dark space is the space which might be used, or might not, depending
-	 * on which node should be written to the LEB. Its watermark is
-	 * equivalent to maximum UBIFS node size.
+	 * Initialize dead and dark LEB space watermarks. See gc.c for comments
+	 * about these values.
 	 */
 	c->dead_wm = ALIGN(MIN_WRITE_SZ, c->min_io_size);
 	c->dark_wm = ALIGN(UBIFS_MAX_NODE_SZ, c->min_io_size);

fs/ubifs/ubifs.h

@@ -426,9 +426,9 @@ struct ubifs_unclean_leb {
  * LEB properties flags.
  *
  * LPROPS_UNCAT: not categorized
- * LPROPS_DIRTY: dirty > 0, not index
+ * LPROPS_DIRTY: dirty > free, dirty >= @c->dead_wm, not index
  * LPROPS_DIRTY_IDX: dirty + free > @c->min_idx_node_sze and index
- * LPROPS_FREE: free > 0, not empty, not index
+ * LPROPS_FREE: free > 0, dirty < @c->dead_wm, not empty, not index
  * LPROPS_HEAP_CNT: number of heaps used for storing categorized LEBs
  * LPROPS_EMPTY: LEB is empty, not taken
  * LPROPS_FREEABLE: free + dirty == leb_size, not index, not taken