On Thu, 2007-12-20 at 14:16 +0000, Hugh Dickins wrote:
> 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.
Ah, yes, I mistakenly assumed it was a reference get. In that case I
stand corrected and do not have any objections.
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