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

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Date:   Fri, 01 Dec 2017 16:02:30 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
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

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