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    SubjectRe: BUG: unable to handle kernel NULL pointer dereference at 0000000000000500
    On 12/28/2012 10:45 AM, Zhouping Liu wrote:
    >> Thank you for the report Zhouping!
    >>
    >> Would you be so kind to test the following patch and report results?
    >> Apply the patch to the latest mainline.
    > Hello Zlatko,
    >
    > I have tested the below patch(applied it on mainline directly),
    > but IMO, I'd like to say it maybe don't fix the issue completely.

    Hi Zlatko,

    I re-tested it on another machine, which has 60+ Gb RAM and 4 numa nodes,
    without your patch, it's easy to reproduce the 'NULL pointer' error,
    after applying your patch, I couldn't reproduce the issue any more.

    depending on the above, it implied that your patch fixed the issue.

    but in my last mail, I tested it on two machines, which caused hung task
    with your patch,
    so I'm confusing is it your patch block some oom-killer performance? if
    it's not, your patch is good for me.

    Thanks,
    Zhouping

    >
    > run the reproducer[1] on two machine, one machine has 2 numa nodes(8Gb RAM),
    > another one has 4 numa nodes(8Gb RAM), then the system hung all the time, such as the dmesg log:
    >
    > [ 713.066937] Killed process 6085 (oom01) total-vm:18880768kB, anon-rss:7915612kB, file-rss:4kB
    > [ 959.555269] INFO: task kworker/13:2:147 blocked for more than 120 seconds.
    > [ 959.562144] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    > [ 1079.382018] INFO: task kworker/13:2:147 blocked for more than 120 seconds.
    > [ 1079.388872] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    > [ 1199.209709] INFO: task kworker/13:2:147 blocked for more than 120 seconds.
    > [ 1199.216562] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    > [ 1319.036939] INFO: task kworker/13:2:147 blocked for more than 120 seconds.
    > [ 1319.043794] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    > [ 1438.864797] INFO: task kworker/13:2:147 blocked for more than 120 seconds.
    > [ 1438.871649] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    > [ 1558.691611] INFO: task kworker/13:2:147 blocked for more than 120 seconds.
    > [ 1558.698466] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    > ......
    >
    > I'm not sure whether it's your patch triggering the hung task or not, but reverted cda73a10eb3,
    > the reproducer(oom01) can PASS without both 'NULL pointer dereference at 0000000000000500' and hung task issues.
    >
    > but some time, it's possible that the reproducer(oom01) cause hung task on a box with large RAM(100Gb+), so I can't judge it...
    >
    > Thanks,
    > Zhouping
    >
    >> Thanks,
    >>
    >> diff --git a/mm/vmscan.c b/mm/vmscan.c
    >> index 23291b9..e55ce55 100644
    >> --- a/mm/vmscan.c
    >> +++ b/mm/vmscan.c
    >> @@ -2564,6 +2564,7 @@ static bool prepare_kswapd_sleep(pg_data_t
    >> *pgdat, int order, long remaining,
    >> static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
    >> int *classzone_idx)
    >> {
    >> + bool pgdat_is_balanced = false;
    >> struct zone *unbalanced_zone;
    >> int i;
    >> int end_zone = 0; /* Inclusive. 0 = ZONE_DMA */
    >> @@ -2638,8 +2639,11 @@ loop_again:
    >> zone_clear_flag(zone, ZONE_CONGESTED);
    >> }
    >> }
    >> - if (i < 0)
    >> +
    >> + if (i < 0) {
    >> + pgdat_is_balanced = true;
    >> goto out;
    >> + }
    >>
    >> for (i = 0; i <= end_zone; i++) {
    >> struct zone *zone = pgdat->node_zones + i;
    >> @@ -2766,8 +2770,11 @@ loop_again:
    >> pfmemalloc_watermark_ok(pgdat))
    >> wake_up(&pgdat->pfmemalloc_wait);
    >>
    >> - if (pgdat_balanced(pgdat, order, *classzone_idx))
    >> + if (pgdat_balanced(pgdat, order, *classzone_idx)) {
    >> + pgdat_is_balanced = true;
    >> break; /* kswapd: all done */
    >> + }
    >> +
    >> /*
    >> * OK, kswapd is getting into trouble. Take a nap, then take
    >> * another pass across the zones.
    >> @@ -2775,7 +2782,7 @@ loop_again:
    >> if (total_scanned && (sc.priority < DEF_PRIORITY - 2)) {
    >> if (has_under_min_watermark_zone)
    >> count_vm_event(KSWAPD_SKIP_CONGESTION_WAIT);
    >> - else
    >> + else if (unbalanced_zone)
    >> wait_iff_congested(unbalanced_zone, BLK_RW_ASYNC, HZ/10);
    >> }
    >>
    >> @@ -2788,9 +2795,9 @@ loop_again:
    >> if (sc.nr_reclaimed >= SWAP_CLUSTER_MAX)
    >> break;
    >> } while (--sc.priority >= 0);
    >> -out:
    >>
    >> - if (!pgdat_balanced(pgdat, order, *classzone_idx)) {
    >> +out:
    >> + if (!pgdat_is_balanced) {
    >> cond_resched();
    >>
    >> try_to_freeze();
    >>
    >> --
    >> Zlatko
    >>
    >> --
    >> To unsubscribe, send a message with 'unsubscribe linux-mm' in
    >> the body to majordomo@kvack.org. For more info on Linux MM,
    >> see: http://www.linux-mm.org/ .
    >> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
    >>



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