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


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