Re: [BUGFIX][PATCH] vmscan: don't use return value trick when oom_killer_disabled

[Minchan Kim <minchan.kim@gmail.com>]

On Wed, Sep 01, 2010 at 11:01:43AM +0900, Minchan Kim wrote:
> On Wed, Sep 1, 2010 at 10:55 AM, KOSAKI Motohiro
> <kosaki.motohiro@jp.fujitsu.com> wrote:
> > Hi
> >
> > Thank you for good commenting!
> >
> >
> >> I don't like use oom_killer_disabled directly.
> >> That's because we have wrapper inline functions to handle the
> >> variable(ex, oom_killer_[disable/enable]).
> >> It means we are reluctant to use the global variable directly.
> >> So should we make new function as is_oom_killer_disable?
> >>
> >> I think NO.
> >>
> >> As I read your description, this problem is related to only hibernation.
> >> Since hibernation freezes all processes(include kswapd), this problem
> >> happens. Of course, now oom_killer_disabled is used by only
> >> hibernation. But it can be used others in future(Off-topic : I don't
> >> want it). Others can use it without freezing processes. Then kswapd
> >> can set zone->all_unreclaimable and the problem can't happen.
> >>
> >> So I want to use sc->hibernation_mode which is already used
> >> do_try_to_free_pages instead of oom_killer_disabled.
> >
> > Unfortunatelly, It's impossible. shrink_all_memory() turn on
> > sc->hibernation_mode. but other hibernation caller merely call
> > alloc_pages(). so we don't have any hint.
> >
> Ahh.. True. Sorry for that.
> I will think some better method.
> if I can't find it, I don't mind this patch. :)

It seems that the poblem happens following as. 
(I might miss something since I just read theyour description)

hibernation
oom_disable
alloc_pages
do_try_to_free_pages
        if (scanning_global_lru(sc) && !all_unreclaimable)
                return 1;
If kswapd is not freezed, it would set zone->all_unreclaimable to 1 and then
shrink_zones maybe return true. so alloc_pages could go to _nopage_.
If it is, it's no problem. 
Right?

I think the problem would come from shrink_zones. 
It set false to all_unreclaimable blindly even though shrink_zone can't reclaim
any page. It doesn't make sense. 
How about this?
I think we need this regardless of the problem.
What do you think about?

diff --git a/mm/vmscan.c b/mm/vmscan.c
index d8fd87d..22017b3 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1901,7 +1901,8 @@ static bool shrink_zones(int priority, struct zonelist *zonelist,
                }
 
                shrink_zone(priority, zone, sc);
-               all_unreclaimable = false;
+               if (sc->nr_reclaimed)
+                       all_unreclaimable = false;
        }
        return all_unreclaimable;
 }

-- 
Kind regards,
Minchan Kim