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

[Tetsuo Handa]

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" ?

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.

> +                             continue;
> +                     bitmap_clear(&tmp, ebit, nbits);
> +                     if (tmp == RADIX_TREE_EXCEPTIONAL_ENTRY)
> +                             __radix_tree_delete(root, node, slot);
> +                     else
> +                             rcu_assign_pointer(*slot, (void *)tmp);
> +             } else if (bitmap) {
> +                     nbits = min(end - start + 1, IDA_BITMAP_BITS - bit);
> +
> +                     if (nbits != IDA_BITMAP_BITS)
> +                             bitmap_clear(bitmap->bitmap, bit, nbits);
> +
> +                     if (nbits == IDA_BITMAP_BITS ||
> +                         bitmap_empty(bitmap->bitmap, IDA_BITMAP_BITS)) {
> +                             kfree(bitmap);
> +                             __radix_tree_delete(root, node, slot);
> +                     }
> +             }
> +
> +             /*
> +              * Already reached the last usable ida bitmap, so just return,
> +              * otherwise overflow will happen.
> +              */
> +             if (index == ULONG_MAX / IDA_BITMAP_BITS)
> +                     break;
> +     }
> +}



> +/**
> + * 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);