Re: "BUG: MAX_LOCKDEP_ENTRIES too low" with 6979 "&type->s_umount_key"

[Waiman Long]

On 5/15/20 1:21 AM, Qian Cai wrote:
> Lockdep is screwed here in next-20200514 due to "BUG: MAX_LOCKDEP_ENTRIES too low". One of the traces below pointed to this linux-next commit,
>
> 8c8e824d4ef0 watch_queue: Introduce a non-repeating system-unique superblock ID
>
> which was accidentally just showed up in next-20200514 along with,
>
> 46896d79c514 watch_queue: Add superblock notifications
>
> I did have here,
>
> CONFIG_SB_NOTIFICATIONS=y
> CONFIG_MOUNT_NOTIFICATIONS=y
> CONFIG_FSINFO=y
>
> While MAX_LOCKDEP_ENTRIES is 32768, I noticed there is one type of lock had a lot along,
>
> # grep  'type->s_umount_keyâ /proc/lockdep_chains | wc -l
> 6979

The lock_list table entries are for tracking a lock's forward and 
backward dependencies. The lockdep_chains isn't the right lockdep file 
to look at. Instead, check the lockdep files for entries with the 
maximum BD (backward dependency) + FD (forward dependency). That will 
give you a better view of which locks are consuming most of the 
lock_list entries. Also take a look at lockdep_stats for an overall view 
of how much various table entries are being consumed.

Cheers,
Longman