MT#55283 store metadata head pointer

Put a pointer to the shard itself in the beginning of the buffer. This
facilitates quick lookup.

Change-Id: I0109d77e56afac3189775dd708970288f175dfe6

lib/bufferpool.c

@@ -90,9 +90,17 @@ static struct bpool_shard *bufferpool_new_shard(struct bufferpool *bp) {
 	struct bpool_shard *ret = g_new0(__typeof(*ret), 1);
 	ret->bp = bp;
 	ret->buf = buf;
+	ret->end = buf + BUFFERPOOL_SHARD_SIZE;
+
+	struct bpool_shard **head = buf;
+	*head = ret;
+
+	static_assert(BUFFERPOOL_ALIGN(sizeof(void *)) == BUFFERPOOL_OVERHEAD,
+			"wrong BUFFERPOOL_OVERHEAD size");
+	buf += BUFFERPOOL_ALIGN(sizeof(void *));
+
 	ret->empty = buf;
 	ret->head = buf;
-	ret->end = buf + BUFFERPOOL_SHARD_SIZE;
 
 	RWLOCK_W(&bpool_shards_lock);
 

lib/bufferpool.h

@@ -7,7 +7,7 @@
 #define BUFFERPOOL_ALIGN(x) (((x + BUFFERPOOL_ALIGNMENT - 1) / BUFFERPOOL_ALIGNMENT) * BUFFERPOOL_ALIGNMENT)
 
 #define BUFFERPOOL_SHARD_SIZE (1LL<<24) // 16 MB, must be a power of two
-#define BUFFERPOOL_OVERHEAD (0) // storage space not available
+#define BUFFERPOOL_OVERHEAD BUFFERPOOL_ALIGN(sizeof(void *)) // storage space not available
 
 #define BUFFERPOOL_BOTTOM_MASK (BUFFERPOOL_SHARD_SIZE - 1)
 #define BUFFERPOOL_TOP_MASK (~BUFFERPOOL_BOTTOM_MASK)