bcache: add code comments for bset.c

This patch tries to add code comments in bset.c, to make some
tricky code and designment to be more comprehensible. Most information
of this patch comes from the discussion between Kent and I, he
offers very informative details. If there is any mistake
of the idea behind the code, no doubt that's from me misrepresentation.
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/bset.c

@@ -366,6 +366,10 @@ EXPORT_SYMBOL(bch_btree_keys_init);
 
 /* Binary tree stuff for auxiliary search trees */
 
+/*
+ * return array index next to j when does in-order traverse
+ * of a binary tree which is stored in a linear array
+ */
 static unsigned inorder_next(unsigned j, unsigned size)
 {
 	if (j * 2 + 1 < size) {
@@ -379,6 +383,10 @@ static unsigned inorder_next(unsigned j, unsigned size)
 	return j;
 }
 
+/*
+ * return array index previous to j when does in-order traverse
+ * of a binary tree which is stored in a linear array
+ */
 static unsigned inorder_prev(unsigned j, unsigned size)
 {
 	if (j * 2 < size) {
@@ -421,6 +429,10 @@ static unsigned __to_inorder(unsigned j, unsigned size, unsigned extra)
 	return j;
 }
 
+/*
+ * Return the cacheline index in bset_tree->data, where j is index
+ * from a linear array which stores the auxiliar binary tree
+ */
 static unsigned to_inorder(unsigned j, struct bset_tree *t)
 {
 	return __to_inorder(j, t->size, t->extra);
@@ -441,6 +453,10 @@ static unsigned __inorder_to_tree(unsigned j, unsigned size, unsigned extra)
 	return j;
 }
 
+/*
+ * Return an index from a linear array which stores the auxiliar binary
+ * tree, j is the cacheline index of t->data.
+ */
 static unsigned inorder_to_tree(unsigned j, struct bset_tree *t)
 {
 	return __inorder_to_tree(j, t->size, t->extra);
@@ -546,6 +562,20 @@ static inline uint64_t shrd128(uint64_t high, uint64_t low, uint8_t shift)
 	return low;
 }
 
+/*
+ * Calculate mantissa value for struct bkey_float.
+ * If most significant bit of f->exponent is not set, then
+ *  - f->exponent >> 6 is 0
+ *  - p[0] points to bkey->low
+ *  - p[-1] borrows bits from KEY_INODE() of bkey->high
+ * if most isgnificant bits of f->exponent is set, then
+ *  - f->exponent >> 6 is 1
+ *  - p[0] points to bits from KEY_INODE() of bkey->high
+ *  - p[-1] points to other bits from KEY_INODE() of
+ *    bkey->high too.
+ * See make_bfloat() to check when most significant bit of f->exponent
+ * is set or not.
+ */
 static inline unsigned bfloat_mantissa(const struct bkey *k,
 				       struct bkey_float *f)
 {
@@ -570,6 +600,16 @@ static void make_bfloat(struct bset_tree *t, unsigned j)
 	BUG_ON(m < l || m > r);
 	BUG_ON(bkey_next(p) != m);
 
+	/*
+	 * If l and r have different KEY_INODE values (different backing
+	 * device), f->exponent records how many least significant bits
+	 * are different in KEY_INODE values and sets most significant
+	 * bits to 1 (by +64).
+	 * If l and r have same KEY_INODE value, f->exponent records
+	 * how many different bits in least significant bits of bkey->low.
+	 * See bfloat_mantiss() how the most significant bit of
+	 * f->exponent is used to calculate bfloat mantissa value.
+	 */
 	if (KEY_INODE(l) != KEY_INODE(r))
 		f->exponent = fls64(KEY_INODE(r) ^ KEY_INODE(l)) + 64;
 	else
@@ -633,6 +673,15 @@ void bch_bset_init_next(struct btree_keys *b, struct bset *i, uint64_t magic)
 }
 EXPORT_SYMBOL(bch_bset_init_next);
 
+/*
+ * Build auxiliary binary tree 'struct bset_tree *t', this tree is used to
+ * accelerate bkey search in a btree node (pointed by bset_tree->data in
+ * memory). After search in the auxiliar tree by calling bset_search_tree(),
+ * a struct bset_search_iter is returned which indicates range [l, r] from
+ * bset_tree->data where the searching bkey might be inside. Then a followed
+ * linear comparison does the exact search, see __bch_bset_search() for how
+ * the auxiliary tree is used.
+ */
 void bch_bset_build_written_tree(struct btree_keys *b)
 {
 	struct bset_tree *t = bset_tree_last(b);
@@ -898,6 +947,17 @@ static struct bset_search_iter bset_search_tree(struct bset_tree *t,
 	unsigned inorder, j, n = 1;
 
 	do {
+		/*
+		 * A bit trick here.
+		 * If p < t->size, (int)(p - t->size) is a minus value and
+		 * the most significant bit is set, right shifting 31 bits
+		 * gets 1. If p >= t->size, the most significant bit is
+		 * not set, right shifting 31 bits gets 0.
+		 * So the following 2 lines equals to
+		 *	if (p >= t->size)
+		 *		p = 0;
+		 * but a branch instruction is avoided.
+		 */
 		unsigned p = n << 4;
 		p &= ((int) (p - t->size)) >> 31;
 
@@ -907,6 +967,9 @@ static struct bset_search_iter bset_search_tree(struct bset_tree *t,
 		f = &t->tree[j];
 
 		/*
+		 * Similar bit trick, use subtract operation to avoid a branch
+		 * instruction.
+		 *
 		 * n = (f->mantissa > bfloat_mantissa())
 		 *	? j * 2
 		 *	: j * 2 + 1;