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