Re: [Qemu-devel] [PATCH v20 0/7] Virtio-balloon Enhancement

[Wang, Wei W]

On Wednesday, December 20, 2017 8:26 PM, Matthew Wilcox wrote:
> On Wed, Dec 20, 2017 at 06:34:36PM +0800, Wei Wang wrote:
> > On 12/19/2017 10:05 PM, Tetsuo Handa wrote:
> > > I think xb_find_set() has a bug in !node path.
> >
> > I think we can probably remove the "!node" path for now. It would be
> > good to get the fundamental part in first, and leave optimization to
> > come as separate patches with corresponding test cases in the future.
> 
> You can't remove the !node path.  You'll see !node when the highest set bit
> is less than 1024.  So do something like this:
> 
>       unsigned long bit;
>       xb_preload(GFP_KERNEL);
>       xb_set_bit(xb, 700);
>       xb_preload_end();
>       bit = xb_find_set(xb, ULONG_MAX, 0);
>       assert(bit == 700);

This above test will result in "!node with bitmap !=NULL", and it goes to the 
regular "if (bitmap)" path, which finds 700.

A better test would be
...
xb_set_bit(xb, 700);
assert(xb_find_set(xb, ULONG_MAX, 800) == ULONG_MAX);
...

The first try with the "if (bitmap)" path doesn't find a set bit, and the 
remaining tries will always result in "!node && !bitmap", which implies no set 
bit anymore and no need to try in this case.

So, I think we can change it to

If (!node && !bitmap)
        return size;


Best,
Wei