[PATCH] ida: document IDA_BITMAP_LONGS calculation

[Namhyung Kim]

IDA_BITMAP_LONGS value is calculated take into account struct ida_bitmap
not to waste memory space. Comment it.

Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxx>
---
 include/linux/idr.h |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/include/linux/idr.h b/include/linux/idr.h
index e968db7..7807e4b 100644
--- a/include/linux/idr.h
+++ b/include/linux/idr.h
@@ -119,9 +119,16 @@ void idr_init(struct idr *idp);
  * pointer isn't necessary.
  */
 #define IDA_CHUNK_SIZE		128	/* 128 bytes per chunk */
+/*
+ * -1 is needed here to fit total chunk size into 128 bytes because
+ * struct ida_bitmap consists of nr_busy and bitmap. Since it is
+ * allocated by kmalloc(), chunk size should be matched with kmalloc
+ * size so that it will not waste memory.
+ */
 #define IDA_BITMAP_LONGS	(128 / sizeof(long) - 1)
-#define IDA_BITMAP_BITS		(IDA_BITMAP_LONGS * sizeof(long) * 8)
+#define IDA_BITMAP_BITS 	(IDA_BITMAP_LONGS * sizeof(long) * 8)
 
+/* IDA bitmap chunk */
 struct ida_bitmap {
 	long			nr_busy;
 	unsigned long		bitmap[IDA_BITMAP_LONGS];
-- 
1.7.2.2

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