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

[Wei Wang]

On 11/30/2017 06:34 PM, Tetsuo Handa wrote:
> Wei Wang wrote:
> > + * @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.

> > +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