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

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

On Thu, Feb 10, 2011 at 12:46 AM, Johannes Weiner <hannes@cmpxchg.org> 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
>
> ---
> 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>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

-- 
Kind regards,
Minchan Kim

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