Re: [PATCH] wrong accounting in direct_remap_pfn_range

[Steven Rostedt <srostedt@redhat.com>]

Keir Fraser wrote:
> 
> 
> On 31/8/06 2:03 am, "Steven Rostedt" <srostedt@redhat.com> wrote:
> 
>>> It's not really missing. We could have a size==0 check *or* we can have the
>>> v!=u check. We don't need both and I think the latter is more obviously
>>> correct, as the test is closer to the code that it 'protects'. Also it's a
>>> fairly idiomatic way of generating and flushing batches of work.
>>>
>> So what is really wrong with this code?  Or is the flushes need even on
>> size == 0?
> 
> This patch is fine. But it's no more correct than the current version of the
> code because there is no bug. I don't think your version is particularly
> clearer.
> 

You're right that the original version was not a bug, which I admittedly 
thought it was.

As for clarity, I'll argue that my version is clearer. Just because it 
shows exactly that nothing needs to be done when size is zero, instead 
having a check for (v != u) that is only true when we have a zero size.

Usually, when I have a condition of that sort that comes after a loop 
incrementing v and lets v return back to u, I have that condition need 
to be done if a) the loop wasn't entered (as in this case), or b) the 
condition in the loop wasn't done at the end (which isn't the case here).

Hence, the reason that I thought it was a bug.  Nowhere is there a 
comment that says /* don't do anything if size was 0 */.

Now enough about clarity.  By the off chance that size would be zero 
(which I still don't know when that would be), we waste time with 
flushing caches and the TLB, not to mention wasted time getting a memory 
page and freeing it.

But this is all moot anyway, since there is no bug being fixed.  But as 
for submitting to mainline, I've submitted enough patches to know that 
Andrew Morton actually cares about readability and optimizations (even 
on the not so often path if it doesn't hurt anything else).  And I have 
a tough time seeing Andrew (or others) think that

   v = u;
   for (i=0; i < size; ...) {
      if (v - u/ ...) {
         ...
         v = u;
         ...
      }
      ...
      v++;
      ...
   }
   if (v != u) { ... }

is more readable and clearer than

   if (!size) return 0;

   for (...) {
     ...
   }
   /* no if needed */
   ....


But that's just my opinion.  Take it for what it's worth.

-- Steve