Re: [PATCH] Move page_assign_page_cgroup to VM_BUG_ON in free_hot_cold_page

[Hugh Dickins <hugh@veritas.com>]

On Thu, 20 Dec 2007, Peter Zijlstra wrote:
> On Thu, 2007-12-20 at 13:14 +0000, Hugh Dickins wrote:
> > On Wed, 19 Dec 2007, Dave Hansen wrote:
> > > > -       page_assign_page_cgroup(page, NULL);
> > > > +       VM_BUG_ON(page_get_page_cgroup(page));
> > > 
> > > Hi Balbir,
> > > 
> > > You generally want to do these like:
> > > 
> > > 	foo = page_assign_page_cgroup(page, NULL);
> > > 	VM_BUG_ON(foo);
> > > 
> > > Some embedded people have been known to optimize kernel size like this:
> > > 
> > > 	#define VM_BUG_ON(x) do{}while(0)
> > 
> > Balbir's patch looks fine to me: I don't get your point there, Dave.
> 
> There was a lengthy discussion here:
>   http://lkml.org/lkml/2007/12/14/131
> 
> on the merit of debug statements with side effects.

Of course, but what's the relevance?

> But looking at our definition:
> 
> #ifdef CONFIG_DEBUG_VM
> #define VM_BUG_ON(cond) BUG_ON(cond)
> #else
> #define VM_BUG_ON(condition) do { } while(0)
> #endif
> 
> disabling CONFIG_DEBUG_VM breaks the code as proposed by Balbir in that
> it will no longer acquire the reference.

But what reference?

struct page_cgroup *page_get_page_cgroup(struct page *page)
{
	return (struct page_cgroup *)
		(page->page_cgroup & ~PAGE_CGROUP_LOCK);
}

I guess the issue is that often a "get" function has a complementary
"put" function, but this isn't one of them.  Would page_page_cgroup
be a better name, perhaps?  I don't know.

Hugh