Re: BUG: unable to handle kernel NULL pointer dereference at 0000000000000500

[Zhouping Liu <zliu@redhat.com>]

> 
> 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.

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
> 
> --
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-- 
Thanks,
Zhouping