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    SubjectRe: [resend][PATCH] mm, vmscan: fix do_try_to_free_pages() livelock
    On Thu, Jun 14, 2012 at 04:13:12AM -0400, kosaki.motohiro@gmail.com wrote:
    > From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    >
    > Currently, do_try_to_free_pages() can enter livelock. Because of,
    > now vmscan has two conflicted policies.
    >
    > 1) kswapd sleep when it couldn't reclaim any page when reaching
    > priority 0. This is because to avoid kswapd() infinite
    > loop. That said, kswapd assume direct reclaim makes enough
    > free pages to use either regular page reclaim or oom-killer.
    > This logic makes kswapd -> direct-reclaim dependency.
    > 2) direct reclaim continue to reclaim without oom-killer until
    > kswapd turn on zone->all_unreclaimble. This is because
    > to avoid too early oom-kill.
    > This logic makes direct-reclaim -> kswapd dependency.
    >
    > In worst case, direct-reclaim may continue to page reclaim forever
    > when kswapd sleeps forever.
    >
    > We can't turn on zone->all_unreclaimable from direct reclaim path
    > because direct reclaim path don't take any lock and this way is racy.
    >
    > Thus this patch removes zone->all_unreclaimable field completely and
    > recalculates zone reclaimable state every time.
    >
    > Note: we can't take the idea that direct-reclaim see zone->pages_scanned
    > directly and kswapd continue to use zone->all_unreclaimable. Because, it
    > is racy. commit 929bea7c71 (vmscan: all_unreclaimable() use
    > zone->all_unreclaimable as a name) describes the detail.
    >
    > Reported-by: Aaditya Kumar <aaditya.kumar.30@gmail.com>
    > Reported-by: Ying Han <yinghan@google.com>
    > Cc: Nick Piggin <npiggin@gmail.com>
    > Acked-by: Rik van Riel <riel@redhat.com>
    > Cc: Michal Hocko <mhocko@suse.cz>
    > Cc: Johannes Weiner <hannes@cmpxchg.org>
    > Cc: Mel Gorman <mel@csn.ul.ie>
    > Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    > Cc: Minchan Kim <minchan.kim@gmail.com>
    > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

    Acked-by: Johannes Weiner <hannes@cmpxchg.org>

    > @@ -2497,12 +2490,11 @@ loop_again:
    > shrink_zone(zone, &sc);
    >
    > reclaim_state->reclaimed_slab = 0;
    > - nr_slab = shrink_slab(&shrink, sc.nr_scanned, lru_pages);
    > + shrink_slab(&shrink, sc.nr_scanned, lru_pages);
    > sc.nr_reclaimed += reclaim_state->reclaimed_slab;
    > total_scanned += sc.nr_scanned;
    >
    > - if (nr_slab == 0 && !zone_reclaimable(zone))
    > - zone->all_unreclaimable = 1;
    > +

    That IS a slight change in behaviour. But then, if you scanned 6
    times the amount of reclaimable pages without freeing a single slab
    page, it's probably not worth going on.


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