linux-kernel - Re: "BUG: MAX_LOCKDEP_ENTRIES too low" with 6979 "&type->s_umount

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Message-ID: <e8c6d3af-3045-0a37-5e9e-bfd60c09f97d@redhat.com>
Date:   Sat, 16 May 2020 21:15:55 -0400
From:   Waiman Long <longman@...hat.com>
To:     Qian Cai <cai@....pw>, Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>
Cc:     David Howells <dhowells@...hat.com>,
        Alexander Viro <viro@...IV.linux.org.uk>,
        linux-fsdevel@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: "BUG: MAX_LOCKDEP_ENTRIES too low" with 6979
 "&type->s_umount_key"

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_key’ /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