Re: [patch] vmscan: fix zone shrinking exit when scan work is done

[Kent Overstreet <kent.overstreet@gmail.com>]

On 02/09/2011 07:46 AM, Johannes Weiner wrote:
> Hi,
>
> I think this should fix the problem of processes getting stuck in
> reclaim that has been reported several times.  Kent actually
> single-stepped through this code and noted that it was never exiting
> shrink_zone(), which really narrowed it down a lot, considering the
> tons of nested loops from the allocator down to the list shrinking.
>
> 	Hannes

I was able to trigger this in just a few minutes stress testing bcache, 
and now it's been going for half an hour working beautifully. Thanks!

>
> ---
> From: Johannes Weiner<hannes@cmpxchg.org>
> Subject: vmscan: fix zone shrinking exit when scan work is done
>
> '3e7d344 mm: vmscan: reclaim order-0 and use compaction instead of
> lumpy reclaim' introduced an indefinite loop in shrink_zone().
>
> It meant to break out of this loop when no pages had been reclaimed
> and not a single page was even scanned.  The way it would detect the
> latter is by taking a snapshot of sc->nr_scanned at the beginning of
> the function and comparing it against the new sc->nr_scanned after the
> scan loop.  But it would re-iterate without updating that snapshot,
> looping forever if sc->nr_scanned changed at least once since
> shrink_zone() was invoked.
>
> This is not the sole condition that would exit that loop, but it
> requires other processes to change the zone state, as the reclaimer
> that is stuck obviously can not anymore.
>
> This is only happening for higher-order allocations, where reclaim is
> run back to back with compaction.
>
> Reported-by: Michal Hocko<mhocko@suse.cz>
> Reported-by: Kent Overstreet<kent.overstreet@gmail.com>
> Signed-off-by: Johannes Weiner<hannes@cmpxchg.org>

Tested-by: Kent Overstreet<kent.overstreet@gmail.com>

> ---
>   mm/vmscan.c |    4 ++--
>   1 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 148c6e6..17497d0 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1882,12 +1882,12 @@ static void shrink_zone(int priority, struct zone *zone,
>   	unsigned long nr[NR_LRU_LISTS];
>   	unsigned long nr_to_scan;
>   	enum lru_list l;
> -	unsigned long nr_reclaimed;
> +	unsigned long nr_reclaimed, nr_scanned;
>   	unsigned long nr_to_reclaim = sc->nr_to_reclaim;
> -	unsigned long nr_scanned = sc->nr_scanned;
>
>   restart:
>   	nr_reclaimed = 0;
> +	nr_scanned = sc->nr_scanned;
>   	get_scan_count(zone, sc, nr, priority);
>
>   	while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||