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SubjectRe: [PATCH v20 3/7 RESEND] xbitmap: add more operations
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Matthew Wilcox wrote:
> > +/**
> > + * xb_find_set - find the next set bit in a range of bits
> > + * @xb: the xbitmap to search from
> > + * @offset: the offset in the range to start searching
> > + * @size: the size of the range
> > + *
> > + * Returns: the found bit or, @size if no set bit is found.
> > + */
> > +unsigned long xb_find_set(struct xb *xb, unsigned long size,
> > + unsigned long offset)
> > +{
> > + struct radix_tree_root *root = &xb->xbrt;
> > + struct radix_tree_node *node;
> > + void __rcu **slot;
> > + struct ida_bitmap *bitmap;
> > + unsigned long index = offset / IDA_BITMAP_BITS;
> > + unsigned long index_end = size / IDA_BITMAP_BITS;
> > + unsigned long bit = offset % IDA_BITMAP_BITS;
> > +
> > + if (unlikely(offset >= size))
> > + return size;
> > +
> > + while (index <= index_end) {
> > + unsigned long ret;
> > + unsigned int nbits = size - index * IDA_BITMAP_BITS;
> > +
> > + bitmap = __radix_tree_lookup(root, index, &node, &slot);
> > +
> > + if (!node && !bitmap)
> > + return size;
> > +
> > + if (bitmap) {
> > + if (nbits > IDA_BITMAP_BITS)
> > + nbits = IDA_BITMAP_BITS;
> > +
> > + ret = find_next_bit(bitmap->bitmap, nbits, bit);
> > + if (ret != nbits)
> > + return ret + index * IDA_BITMAP_BITS;
> > + }
> > + bit = 0;
> > + index++;
> > + }
> > +
> > + return size;
> > +}
> > +EXPORT_SYMBOL(xb_find_set);
>
> This is going to be slower than the implementation I sent yesterday. If I
> call:
> xb_init(xb);
> xb_set_bit(xb, ULONG_MAX);
> xb_find_set(xb, ULONG_MAX, 0);
>
> it's going to call __radix_tree_lookup() 16 quadrillion times.
> My implementation will walk the tree precisely once.
>
Yes. Wei's patch still can not work.
We should start reviewing Matthew's implementation.

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