Re: [BUGFIX][PATCH 1/4] memcg: fix limit estimation at reclaim forhugepage

[Johannes Weiner]

On Fri, Jan 28, 2011 at 05:25:58PM +0900, Minchan Kim wrote:
> Hi Hannes,
> 
> On Fri, Jan 28, 2011 at 5:17 PM, Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> > On Fri, Jan 28, 2011 at 05:04:16PM +0900, Minchan Kim wrote:
> >> Hi Kame,
> >>
> >> On Fri, Jan 28, 2011 at 1:58 PM, KAMEZAWA Hiroyuki
> >> <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> >> > How about this ?
> >> > ==
> >> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> >> >
> >> > Current memory cgroup's code tends to assume page_size == PAGE_SIZE
> >> > and arrangement for THP is not enough yet.
> >> >
> >> > This is one of fixes for supporing THP. This adds
> >> > mem_cgroup_check_margin() and checks whether there are required amount of
> >> > free resource after memory reclaim. By this, THP page allocation
> >> > can know whether it really succeeded or not and avoid infinite-loop
> >> > and hangup.
> >> >
> >> > Total fixes for do_charge()/reclaim memory will follow this patch.
> >>
> >> If this patch is only related to THP, I think patch order isn't good.
> >> Before applying [2/4], huge page allocation will retry without
> >> reclaiming and loop forever by below part.
> >>
> >> @@ -1854,9 +1858,6 @@ static int __mem_cgroup_do_charge(struct
> >>       } else
> >>               mem_over_limit = mem_cgroup_from_res_counter(fail_res, res);
> >>
> >> -     if (csize > PAGE_SIZE) /* change csize and retry */
> >> -             return CHARGE_RETRY;
> >> -
> >>       if (!(gfp_mask & __GFP_WAIT))
> >>               return CHARGE_WOULDBLOCK;
> >>
> >> Am I missing something?
> >
> > No, you are correct.  But I am not sure the order really matters in
> > theory: you have two endless loops that need independent fixing.
> 
> That's why I ask a question.
> Two endless loop?
> 
> One is what I mentioned. The other is what?

The other is

1. huge page charge, fail
2. reclaim
3. limit is ok, retry at 1.

The problem is that the limit will be okay for regular pages, but not
huge pages.

Currently, this code is never reached due to the endless loop you
already spotted.

HTH
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