Re: [PATCH] mm: check zone->all_unreclaimable inall_unreclaimable()

[Andrew Morton]

On Sun, 6 Mar 2011 02:07:59 +0900
Minchan Kim <minchan.kim@xxxxxxxxx> 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@xxxxxxxxxx>
> > >>>>---
> > >>>>  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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