Re: [PATCH v19 3/7] xbitmap: add more operations

[Wei Wang]

On 12/12/2017 09:20 PM, Tetsuo Handa wrote:
> Wei Wang wrote:
> > +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);
> > +
> > +			if (ebit >= BITS_PER_LONG)
> What happens if we hit this "continue;" when "index == ULONG_MAX / IDA_BITMAP_BITS" ?

Thanks. I also improved the test case for this. I plan to change the 
implementation a little bit to avoid such overflow (has passed the test 
case that I have, just post out for another set of eyes):

{
...
        unsigned long idx = start / IDA_BITMAP_BITS;
        unsigned long bit = start % IDA_BITMAP_BITS;
        unsigned long idx_end = end / IDA_BITMAP_BITS;
        unsigned long ret;

        for (idx = start / IDA_BITMAP_BITS; idx <= idx_end; idx++) {
                unsigned long ida_start = idx * IDA_BITMAP_BITS;

                bitmap = __radix_tree_lookup(root, idx, &node, &slot);
                if (radix_tree_exception(bitmap)) {
                        unsigned long tmp = (unsigned long)bitmap;
                        unsigned long ebit = bit + 2;

                        if (ebit >= BITS_PER_LONG)
                                continue;
                        if (set)
                                ret = find_next_bit(&tmp, 
BITS_PER_LONG, ebit);
                        else
                                ret = find_next_zero_bit(&tmp, 
BITS_PER_LONG,
                                                         ebit);
                        if (ret < BITS_PER_LONG)
                                return ret - 2 + ida_start;
                } else if (bitmap) {
                        if (set)
                                ret = find_next_bit(bitmap->bitmap,
                                                    IDA_BITMAP_BITS, bit);
                        else
                                ret = find_next_zero_bit(bitmap->bitmap,
IDA_BITMAP_BITS, bit);
                        if (ret < IDA_BITMAP_BITS)
                                return ret + ida_start;
                } else if (!bitmap && !set) {
                        return bit + IDA_BITMAP_BITS * idx;
                }
                bit = 0;
        }

        return end;
}


> Can you eliminate exception path and fold all xbitmap patches into one, and
> post only one xbitmap patch without virtio-baloon changes? If exception path
> is valuable, you can add exception path after minimum version is merged.
> This series is too difficult for me to close corner cases.

That exception path is claimed to save memory, and I don't have a strong 
reason to remove that part.
Matthew, could we get your feedback on this?



> > +/**
> > + * xb_find_next_set_bit - find the next set bit in a range
> > + * @xb: the xbitmap to search
> > + * @start: the start of the range, inclusive
> > + * @end: the end of the range, exclusive
> > + *
> > + * Returns: the index of the found bit, or @end + 1 if no such bit is found.
> > + */
> > +unsigned long xb_find_next_set_bit(struct xb *xb, unsigned long start,
> > +				   unsigned long end)
> > +{
> > +	return xb_find_next_bit(xb, start, end, 1);
> > +}
> Won't "exclusive" loose ability to handle ULONG_MAX ? Since this is a
> library module, missing ability to handle ULONG_MAX sounds like an omission.
> Shouldn't we pass (or return) whether "found or not" flag (e.g. strtoul() in
> C library function)?
>
>    bool xb_find_next_set_bit(struct xb *xb, unsigned long start, unsigned long end, unsigned long *result);
>    unsigned long xb_find_next_set_bit(struct xb *xb, unsigned long start, unsigned long end, bool *found);

Yes, ULONG_MAX needs to be tested by xb_test_bit(). Compared to checking 
the return value, would it be the same to let the caller check for the 
ULONG_MAX boundary?

Best,
Wei