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

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Date:   Fri, 1 Dec 2017 22:02:01 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     wei.w.wang@...el.com
Cc:     virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        qemu-devel@...gnu.org, virtualization@...ts.linux-foundation.org,
        kvm@...r.kernel.org, linux-mm@...ck.org, mst@...hat.com,
        mhocko@...nel.org, akpm@...ux-foundation.org,
        mawilcox@...rosoft.com, david@...hat.com, cornelia.huck@...ibm.com,
        mgorman@...hsingularity.net, aarcange@...hat.com,
        amit.shah@...hat.com, pbonzini@...hat.com, willy@...radead.org,
        liliang.opensource@...il.com, yang.zhang.wz@...il.com,
        quan.xu@...yun.com, nilal@...hat.com, riel@...hat.com
Subject: Re: [PATCH v18 05/10] xbitmap: add more operations

Wei Wang wrote:
> 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.

If start == end is legal,

   for (; start < end; start = (start | (IDA_BITMAP_BITS - 1)) + 1) {

makes this loop do nothing because 10 < 10 is false.



> 
> >
> >> +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).

I think that

+	else if (__builtin_constant_p(start) && IS_ALIGNED(start, 8) &&
+		 __builtin_constant_p(nbits) && IS_ALIGNED(nbits, 8))

is more readable.