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    SubjectRe: [PATCH] mm: check zone->all_unreclaimable in all_unreclaimable()
    On Sun, 6 Mar 2011 02:07:59 +0900
    Minchan Kim <minchan.kim@gmail.com> wrote:

    > On Sat, Mar 05, 2011 at 07:41:26PM +0300, Andrew Vagin wrote:
    > > On 03/05/2011 06:53 PM, Minchan Kim wrote:
    > > >On Sat, Mar 05, 2011 at 06:34:37PM +0300, Andrew Vagin wrote:
    > > >>On 03/05/2011 06:20 PM, Minchan Kim wrote:
    > > >>>On Sat, Mar 05, 2011 at 02:44:16PM +0300, Andrey Vagin wrote:
    > > >>>>Check zone->all_unreclaimable in all_unreclaimable(), otherwise the
    > > >>>>kernel may hang up, because shrink_zones() will do nothing, but
    > > >>>>all_unreclaimable() will say, that zone has reclaimable pages.
    > > >>>>
    > > >>>>do_try_to_free_pages()
    > > >>>> shrink_zones()
    > > >>>> for_each_zone
    > > >>>> if (zone->all_unreclaimable)
    > > >>>> continue
    > > >>>> if !all_unreclaimable(zonelist, sc)
    > > >>>> return 1
    > > >>>>
    > > >>>>__alloc_pages_slowpath()
    > > >>>>retry:
    > > >>>> did_some_progress = do_try_to_free_pages(page)
    > > >>>> ...
    > > >>>> if (!page&& did_some_progress)
    > > >>>> retry;
    > > >>>>
    > > >>>>Signed-off-by: Andrey Vagin<avagin@openvz.org>
    > > >>>>---
    > > >>>> mm/vmscan.c | 2 ++
    > > >>>> 1 files changed, 2 insertions(+), 0 deletions(-)
    > > >>>>
    > > >>>>diff --git a/mm/vmscan.c b/mm/vmscan.c
    > > >>>>index 6771ea7..1c056f7 100644
    > > >>>>--- a/mm/vmscan.c
    > > >>>>+++ b/mm/vmscan.c
    > > >>>>@@ -2002,6 +2002,8 @@ static bool all_unreclaimable(struct zonelist *zonelist,
    > > >>>>
    > > >>>> for_each_zone_zonelist_nodemask(zone, z, zonelist,
    > > >>>> gfp_zone(sc->gfp_mask), sc->nodemask) {
    > > >>>>+ if (zone->all_unreclaimable)
    > > >>>>+ continue;
    > > >>>> if (!populated_zone(zone))
    > > >>>> continue;
    > > >>>> if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
    > > >>>zone_reclaimable checks it. Isn't it enough?
    > > >>I sent one more patch [PATCH] mm: skip zombie in OOM-killer.
    > > >>This two patches are enough.
    > > >Sorry if I confused you.
    > > >I mean zone->all_unreclaimable become true if !zone_reclaimable in balance_pgdat.
    > > >zone_reclaimable compares recent pages_scanned with the number of zone lru pages.
    > > >So too many page scanning in small lru pages makes the zone to unreclaimable zone.
    > > >
    > > >In all_unreclaimable, we calls zone_reclaimable to detect it.
    > > >It's the same thing with your patch.
    > > balance_pgdat set zone->all_unreclaimable, but the problem is that
    > > it is cleaned late.
    >
    > Yes. It can be delayed by pcp so (zone->all_unreclaimable = true) is
    > a false alram since zone have a free page and it can be returned
    > to free list by drain_all_pages in next turn.
    >
    > >
    > > The problem is that zone->all_unreclaimable = True, but
    > > zone_reclaimable() returns True too.
    >
    > Why is it a problem?
    > If zone->all_unreclaimable gives a false alram, we does need to check
    > it again by zone_reclaimable call.
    >
    > If we believe a false alarm and give up the reclaim, maybe we have to make
    > unnecessary oom kill.
    >
    > >
    > > zone->all_unreclaimable will be cleaned in free_*_pages, but this
    > > may be late. It is enough allocate one page from page cache, that
    > > zone_reclaimable() returns True and zone->all_unreclaimable becomes
    > > True.
    > > >>>Does the hang up really happen or see it by code review?
    > > >>Yes. You can reproduce it for help the attached python program. It's
    > > >>not very clever:)
    > > >>It make the following actions in loop:
    > > >>1. fork
    > > >>2. mmap
    > > >>3. touch memory
    > > >>4. read memory
    > > >>5. munmmap
    > > >It seems the test program makes fork bombs and memory hogging.
    > > >If you applied this patch, the problem is gone?
    > > Yes.
    >
    > Hmm.. Although it solves the problem, I think it's not a good idea that
    > depends on false alram and give up the retry.

    Any alternative proposals? We should get the livelock fixed if possible..


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