Re: [PATCH] memcg: reset to root_mem_cgroup at bypassing

[Hugh Dickins <hughd@google.com>]

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On Wed, 21 Dec 2011, KAMEZAWA Hiroyuki wrote:
> On Tue, 20 Dec 2011 09:24:47 +0900
> Hiroyuki Kamezawa <kamezawa.hiroyuki@gmail.com> wrote:
> > 2011/12/20 Hugh Dickins <hughd@google.com>:
> > 
> > > I speak from experience: I did *exactly* the same at "bypass" when
> > > I introduced our mem_cgroup_reset_page(), which corresponds to your
> > > mem_cgroup_reset_owner(); it seemed right to me that a successful
> > > (return 0) call to try_charge() should provide a good *ptr.
> > >
> > ok.
> > 
> > > But others (Ying and Greg) pointed out that it changes the semantics
> > > of __mem_cgroup_try_charge() in this case, so you need to justify the
> > > change to all those places which do something like "if (ret || !memcg)"
> > > after calling it.  Perhaps it is a good change everywhere, but that's
> > > not obvious, so we chose caution.
> > >
> > > Doesn't it lead to bypass pages being marked as charged to root, so
> > > they don't get charged to the right owner next time they're touched?
> > >
> > Yes. You're right.
> > Hm. So, it seems I should add reset_owner() to the !memcg path
> > rather than here.
> > 
> Considering this again..
> 
> Now, we catch 'charge' event only once in lifetime of anon/file page.
> So, it doesn't depend on that it's marked as PCG_USED or not.

That's an interesting argument, I hadn't been looking at it that way.
It's not true of swapcache, but I guess we don't need to preserve its
peculiarities in this case.

I've not checked the (ret || !memcg) cases yet to see if any change
needed there.

I certainly like that the success return guarantees that memcg is set.

Hugh