Re: [PATCH v2 07/11] mm: Allocate kernel pages to the right memcg

[Glauber Costa <glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>]

On 08/15/2012 05:22 PM, Mel Gorman wrote:
>> I believe it
>> > to be a better and less complicated approach then letting a page appear
>> > and then charging it. Besides being consistent with the rest of memcg,
>> > it won't create unnecessary disturbance in the page allocator
>> > when the allocation is to fail.
>> > 
> I still don't get why you did not just return a mem_cgroup instead of a
> handle.
> 

Forgot this one, sorry:

The reason is to keep the semantics simple.

What should we return if the code is not compiled in? If we return NULL
for failure, the test becomes

memcg = memcg_kmem_charge_page(gfp, order);
if (!memcg)
  exit;

If we're not compiled in, we'd either return positive garbage or we need
to wrap it inside an ifdef

I personally believe to be a lot more clear to standardize on true
to mean "allocation can proceed".

the compiled out case becomes:

if (!true)
   exit;

which is easily compiled away altogether. Now of course, using struct
mem_cgroup makes sense, and I have already changed that here.