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    Subject[PATCH 2/3] mm, vmscan: consider eligible zones in get_scan_count
    Date
    From: Michal Hocko <mhocko@suse.com>

    get_scan_count considers the whole node LRU size when
    - doing SCAN_FILE due to many page cache inactive pages
    - calculating the number of pages to scan

    in both cases this might lead to unexpected behavior especially on 32b
    systems where we can expect lowmem memory pressure very often.

    A large highmem zone can easily distort SCAN_FILE heuristic because
    there might be only few file pages from the eligible zones on the node
    lru and we would still enforce file lru scanning which can lead to
    trashing while we could still scan anonymous pages.

    The later use of lruvec_lru_size can be problematic as well. Especially
    when there are not many pages from the eligible zones. We would have to
    skip over many pages to find anything to reclaim but shrink_node_memcg
    would only reduce the remaining number to scan by SWAP_CLUSTER_MAX
    at maximum. Therefore we can end up going over a large LRU many times
    without actually having chance to reclaim much if anything at all. The
    closer we are out of memory on lowmem zone the worse the problem will
    be.

    Fix this by filtering out all the ineligible zones when calculating the
    lru size for both paths and consider only sc->reclaim_idx zones.

    Acked-by: Minchan Kim <minchan@kernel.org>
    Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
    Signed-off-by: Michal Hocko <mhocko@suse.com>
    ---
    mm/vmscan.c | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    diff --git a/mm/vmscan.c b/mm/vmscan.c
    index aed39dc272c0..ffac8fa7bdd8 100644
    --- a/mm/vmscan.c
    +++ b/mm/vmscan.c
    @@ -2235,7 +2235,7 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
    * system is under heavy pressure.
    */
    if (!inactive_list_is_low(lruvec, true, sc, false) &&
    - lruvec_lru_size(lruvec, LRU_INACTIVE_FILE, MAX_NR_ZONES) >> sc->priority) {
    + lruvec_lru_size(lruvec, LRU_INACTIVE_FILE, sc->reclaim_idx) >> sc->priority) {
    scan_balance = SCAN_FILE;
    goto out;
    }
    @@ -2302,7 +2302,7 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
    unsigned long size;
    unsigned long scan;

    - size = lruvec_lru_size(lruvec, lru, MAX_NR_ZONES);
    + size = lruvec_lru_size(lruvec, lru, sc->reclaim_idx);
    scan = size >> sc->priority;

    if (!scan && pass && force_scan)
    --
    2.11.0
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