Re: [PATCH 1/5] vmscan: remove all_unreclaimable check from directreclaim path completely

[Minchan Kim]

Hi Kosaki,

On Tue, Mar 22, 2011 at 08:05:55PM +0900, KOSAKI Motohiro wrote:
> all_unreclaimable check in direct reclaim has been introduced at 2.6.19
> by following commit.
> 
> 	2006 Sep 25; commit 408d8544; oom: use unreclaimable info
> 
> And it went through strange history. firstly, following commit broke
> the logic unintentionally.
> 
> 	2008 Apr 29; commit a41f24ea; page allocator: smarter retry of
> 				      costly-order allocations
> 
> Two years later, I've found obvious meaningless code fragment and
> restored original intention by following commit.
> 
> 	2010 Jun 04; commit bb21c7ce; vmscan: fix do_try_to_free_pages()
> 				      return value when priority==0
> 
> But, the logic didn't works when 32bit highmem system goes hibernation
> and Minchan slightly changed the algorithm and fixed it .
> 
> 	2010 Sep 22: commit d1908362: vmscan: check all_unreclaimable
> 				      in direct reclaim path
> 
> But, recently, Andrey Vagin found the new corner case. Look,
> 
> 	struct zone {
> 	  ..
> 	        int                     all_unreclaimable;
> 	  ..
> 	        unsigned long           pages_scanned;
> 	  ..
> 	}
> 
> zone->all_unreclaimable and zone->pages_scanned are neigher atomic
> variables nor protected by lock. Therefore a zone can become a state
> of zone->page_scanned=0 and zone->all_unreclaimable=1. In this case,

Possible although it's very rare.

> current all_unreclaimable() return false even though
> zone->all_unreclaimabe=1.

The case is very rare since we reset zone->all_unreclaimabe to zero
right before resetting zone->page_scanned to zero.
But I admit it's possible.

        CPU 0                                           CPU 1
free_pcppages_bulk                              balance_pgdat
        zone->all_unreclaimabe = 0
                                                        zone->all_unreclaimabe = 1
        zone->pages_scanned = 0
> 
> Is this ignorable minor issue? No. Unfortunatelly, x86 has very
> small dma zone and it become zone->all_unreclamble=1 easily. and
> if it becase all_unreclaimable, it never return all_unreclaimable=0
        ^^^^^ it's very important verb.    ^^^^^ return? reset?

        I can't understand your point due to the typo. Please correct the typo.

> beucase it typicall don't have reclaimable pages.

If DMA zone have very small reclaimable pages or zero reclaimable pages,
zone_reclaimable() can return false easily so all_unreclaimable() could return
true. Eventually oom-killer might works.

In my test, I saw the livelock, too so apparently we have a problem.
I couldn't dig in it recently by another urgent my work.
I think you know root cause but the description in this patch isn't enough
for me to be persuaded.

Could you explain the root cause in detail?

> 
> Eventually, oom-killer never works on such systems.  Let's remove
> this problematic logic completely.
> 
> Reported-by: Andrey Vagin <avagin@xxxxxxxxxx>
> Cc: Nick Piggin <npiggin@xxxxxxxxx>
> Cc: Minchan Kim <minchan.kim@xxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> ---
>  mm/vmscan.c |   36 +-----------------------------------
>  1 files changed, 1 insertions(+), 35 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 060e4c1..254aada 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1989,33 +1989,6 @@ static bool zone_reclaimable(struct zone *zone)
>  }
>  
>  /*
> - * As hibernation is going on, kswapd is freezed so that it can't mark
> - * the zone into all_unreclaimable. It can't handle OOM during hibernation.
> - * So let's check zone's unreclaimable in direct reclaim as well as kswapd.
> - */
> -static bool all_unreclaimable(struct zonelist *zonelist,
> -		struct scan_control *sc)
> -{
> -	struct zoneref *z;
> -	struct zone *zone;
> -	bool all_unreclaimable = true;
> -
> -	for_each_zone_zonelist_nodemask(zone, z, zonelist,
> -			gfp_zone(sc->gfp_mask), sc->nodemask) {
> -		if (!populated_zone(zone))
> -			continue;
> -		if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
> -			continue;
> -		if (zone_reclaimable(zone)) {
> -			all_unreclaimable = false;
> -			break;
> -		}
> -	}
> -
> -	return all_unreclaimable;
> -}
> -
> -/*
>   * This is the main entry point to direct page reclaim.
>   *
>   * If a full scan of the inactive list fails to free enough memory then we
> @@ -2105,14 +2078,7 @@ out:
>  	delayacct_freepages_end();
>  	put_mems_allowed();
>  
> -	if (sc->nr_reclaimed)
> -		return sc->nr_reclaimed;
> -
> -	/* top priority shrink_zones still had more to do? don't OOM, then */
> -	if (scanning_global_lru(sc) && !all_unreclaimable(zonelist, sc))
> -		return 1;
> -
> -	return 0;
> +	return sc->nr_reclaimed;
>  }
>  
>  unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
> -- 
> 1.6.5.2
> 
> 
> 

-- 
Kind regards,
Minchan Kim
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