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    Subject[PATCH 3.18 24/33] mlock: fix mlock count can not decrease in race condition
    Date
    3.18-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Yisheng Xie <xieyisheng1@huawei.com>

    commit 70feee0e1ef331b22cc51f383d532a0d043fbdcc upstream.

    Kefeng reported that when running the follow test, the mlock count in
    meminfo will increase permanently:

    [1] testcase
    linux:~ # cat test_mlockal
    grep Mlocked /proc/meminfo
    for j in `seq 0 10`
    do
    for i in `seq 4 15`
    do
    ./p_mlockall >> log &
    done
    sleep 0.2
    done
    # wait some time to let mlock counter decrease and 5s may not enough
    sleep 5
    grep Mlocked /proc/meminfo

    linux:~ # cat p_mlockall.c
    #include <sys/mman.h>
    #include <stdlib.h>
    #include <stdio.h>

    #define SPACE_LEN 4096

    int main(int argc, char ** argv)
    {
    int ret;
    void *adr = malloc(SPACE_LEN);
    if (!adr)
    return -1;

    ret = mlockall(MCL_CURRENT | MCL_FUTURE);
    printf("mlcokall ret = %d\n", ret);

    ret = munlockall();
    printf("munlcokall ret = %d\n", ret);

    free(adr);
    return 0;
    }

    In __munlock_pagevec() we should decrement NR_MLOCK for each page where
    we clear the PageMlocked flag. Commit 1ebb7cc6a583 ("mm: munlock: batch
    NR_MLOCK zone state updates") has introduced a bug where we don't
    decrement NR_MLOCK for pages where we clear the flag, but fail to
    isolate them from the lru list (e.g. when the pages are on some other
    cpu's percpu pagevec). Since PageMlocked stays cleared, the NR_MLOCK
    accounting gets permanently disrupted by this.

    Fix it by counting the number of page whose PageMlock flag is cleared.

    Fixes: 1ebb7cc6a583 (" mm: munlock: batch NR_MLOCK zone state updates")
    Link: http://lkml.kernel.org/r/1495678405-54569-1-git-send-email-xieyisheng1@huawei.com
    Signed-off-by: Yisheng Xie <xieyisheng1@huawei.com>
    Reported-by: Kefeng Wang <wangkefeng.wang@huawei.com>
    Tested-by: Kefeng Wang <wangkefeng.wang@huawei.com>
    Cc: Vlastimil Babka <vbabka@suse.cz>
    Cc: Joern Engel <joern@logfs.org>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: Michel Lespinasse <walken@google.com>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Michal Hocko <mhocko@suse.cz>
    Cc: Xishi Qiu <qiuxishi@huawei.com>
    Cc: zhongjiang <zhongjiang@huawei.com>
    Cc: Hanjun Guo <guohanjun@huawei.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    mm/mlock.c | 5 +++--
    1 file changed, 3 insertions(+), 2 deletions(-)

    --- a/mm/mlock.c
    +++ b/mm/mlock.c
    @@ -333,7 +333,7 @@ static void __munlock_pagevec(struct pag
    {
    int i;
    int nr = pagevec_count(pvec);
    - int delta_munlocked;
    + int delta_munlocked = -nr;
    struct pagevec pvec_putback;
    int pgrescued = 0;

    @@ -353,6 +353,8 @@ static void __munlock_pagevec(struct pag
    continue;
    else
    __munlock_isolation_failed(page);
    + } else {
    + delta_munlocked++;
    }

    /*
    @@ -364,7 +366,6 @@ static void __munlock_pagevec(struct pag
    pagevec_add(&pvec_putback, pvec->pages[i]);
    pvec->pages[i] = NULL;
    }
    - delta_munlocked = -nr + pagevec_count(&pvec_putback);
    __mod_zone_page_state(zone, NR_MLOCK, delta_munlocked);
    spin_unlock_irq(&zone->lru_lock);


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