Re: [Xen-devel] [PATCH] slightly consolidate code in free_domheap_pages()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.06.14 at 14:10, <Ian.Campbell@xxxxxxxxxxxxx> wrote:
> On Tue, 2014-06-24 at 12:53 +0100, Jan Beulich wrote:
>> >>> On 24.06.14 at 13:27, <Ian.Campbell@xxxxxxxxxxxxx> wrote:
>> > On Tue, 2014-06-24 at 11:25 +0100, Jan Beulich wrote:
>> >> >>> On 24.06.14 at 12:04, <Ian.Campbell@xxxxxxxxxxxxx> wrote:
>> >> > If so, can d at this point ever be anything other than dom_cow or NULL?
>> >> > I don't think so. Given that I think ASSERT(!(d == dom_cow && order !=
>> >> > 0)) would more clearly capture the intent of the test (with the spelling
>> >> > out of the conditions being more important than the de morganing of the
>> >> > expression).
>> >> 
>> >> Indeed, d can only be NULL or dom_cow here (being in the else part
>> >> of the if() you quoted at the top). So an alternative might indeed be
>> >> ASSERT(d != dom_cow || !order), but that seems less desirable to
>> >> me as it opens up ways to pass the ASSERT() with d != NULL should
>> >> the if() condition ever get modified. I.e. I'd prefer the assertion to be
>> >> as restrictive as possible, getting relaxed only when in fact necessary.
>> > 
>> > Since the original if involves d == dom_cow but nothing to do with order
>> > it seemed that the check was somehow specific to dom_cow's relationship
>> > to higher order allocations.
>> > 
>> > I suppose the question is what relationship would a non-NULL d have to
>> > the order of the allocation. i.e. if the if were changed to also
>> > consider dom_foo why would we expect now that dom_foo had any order
>> > requirements?
>> 
>> We won't know, but by having it the way it is now in the patch we're
>> on the safe side (nothing unintended will slip through), whereas if we
>> change to comparing against dom_cow a not sufficiently careful future
>> change may introduce an issue.
> 
> That's true I . Could you add a comment though, since as it is the
> current relationship to dom_cow and order is somewhat obscured.

That I can do, ...

> ASSERT(d == NULL || ( d == dom_cow && !order ) ) is too much?

... but this looks too ugly to me (namely in the else path to an if
checking d against exactly these two values).

Jan


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