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Re: [libvirt] [PATCH] Add VIR_DIV_UP to divide memory or storage request sizes with round up



[replying to myself - never a good sign on Friday afternoon]

On 01/28/2011 02:59 PM, Eric Blake wrote:
>> +/* divide value by size, rounding up */
>> +# define VIR_DIV_UP(value, size) (((value) + (size) - 1) / (size))
> 
> Hmm - now that we are codifying it, can we make it less prone to
> overflow issues?

> ACK to everything else, but let's make sure we agree on the final
> version of VIR_DIV_UP being committed.

One possible issue is that pre-patch [if value is int and size is a
power of 2], then INT_MAX / size * size is nicely truncated to INT_MAX &
~size, while post-patch, VIR_DIV_UP(INT_MAX, size) * size overflows.
That is, while we can tweak your patch to avoid overflow on the division
side, it doesn't affect overflow on the re-multiplication side, so
avoiding overflow is a bigger audit anyways.

That, and you've refactored it, so if we change our minds, it's now a
single line to change (rather than every caller of VIR_DIV_UP being
open-coded).

So let's push it as-is (but with an additional tweak to cover the new
divisions in 'virsh freecell --all' that were added in the meantime).

-- 
Eric Blake   eblake redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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