Re: [Qemu-devel] [PATCH v20 3/7 RESEND] xbitmap: add more operations

[Tetsuo Handa]

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.