Re: [PATCH v18 05/10] xbitmap: add more operations

On 11/30/2017 06:34 PM, Tetsuo Handa wrote:

Wei Wang wrote:

quoted

+ * @start: the start of the bit range, inclusive
+ * @end: the end of the bit range, inclusive
+ *
+ * This function is used to clear a bit in the xbitmap. If all the bits of the
+ * bitmap are 0, the bitmap will be freed.
+ */
+void xb_clear_bit_range(struct xb *xb, unsigned long start, unsigned long end)
+{
+	struct radix_tree_root *root = &xb->xbrt;
+	struct radix_tree_node *node;
+	void **slot;
+	struct ida_bitmap *bitmap;
+	unsigned int nbits;
+
+	for (; start < end; start = (start | (IDA_BITMAP_BITS - 1)) + 1) {
+		unsigned long index = start / IDA_BITMAP_BITS;
+		unsigned long bit = start % IDA_BITMAP_BITS;
+
+		bitmap = __radix_tree_lookup(root, index, &node, &slot);
+		if (radix_tree_exception(bitmap)) {
+			unsigned long ebit = bit + 2;
+			unsigned long tmp = (unsigned long)bitmap;
+
+			nbits = min(end - start + 1, BITS_PER_LONG - ebit);

"nbits = min(end - start + 1," seems to expect that start == end is legal
for clearing only 1 bit. But this function is no-op if start == end.
Please clarify what "inclusive" intended.

If xb_clear_bit_range(xb,10,10), then it is effectively the same as 
xb_clear_bit(10). Why would it be illegal?

"@start inclusive" means that the @start will also be included to be 
cleared.

quoted

+static inline __always_inline void bitmap_clear(unsigned long *map,
+						unsigned int start,
+						unsigned int nbits)
+{
+	if (__builtin_constant_p(nbits) && nbits == 1)
+		__clear_bit(start, map);
+	else if (__builtin_constant_p(start & 7) && IS_ALIGNED(start, 8) &&
+		 __builtin_constant_p(nbits & 7) && IS_ALIGNED(nbits, 8))

It looks strange to apply __builtin_constant_p test to variables after "& 7".

I think this is normal - if the variables are known at compile time, the 
calculation will be done at compile time (termed constant folding).


Best,
Wei