linux-kernel - Re: [PATCH] ocfs2: reflink deadlock when clone file to the same directory simultaneously

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Date:   Wed, 18 Aug 2021 17:20:53 +0800
From:   Gang He <ghe@...e.com>
To:     Joseph Qi <joseph.qi@...ux.alibaba.com>, mark@...heh.com,
        jlbec@...lplan.org, Wengang Wang <wen.gang.wang@...cle.com>
CC:     linux-kernel@...r.kernel.org, ocfs2-devel@....oracle.com,
        akpm@...ux-foundation.org
Subject: Re: [PATCH] ocfs2: reflink deadlock when clone file to the same
 directory simultaneously



On 2021/8/13 17:54, Joseph Qi wrote:
> 
> 
> On 8/9/21 6:08 PM, Gang He wrote:
>> Hi Joseph and All,
>>
>> The deadlock is caused by self-locking on one node.
>> There is three node cluster (mounted to /mnt/shared), the user run reflink command to clone the file to the same directory repeatedly,
>> e.g.
>>   reflink "/mnt/shared/test" \
>>   "/mnt/shared/.snapshots/test.`date +%m%d%H%M%S`.`hostname`"
>>
>> After a while, the reflink process on each node is hung, the file system cannot be listed.
>> The problematic reflink command process is blocked by itself, e.g. the reflink process is hung at ghe-sle15sp2-nd2,
>> kernel: task:reflink         state:D stack:    0 pid:16992 ppid:  4530
>> kernel: Call Trace:
>> kernel:  __schedule+0x2fd/0x750
>> kernel:  ? try_to_wake_up+0x17b/0x4e0
>> kernel:  schedule+0x2f/0xa0
>> kernel:  schedule_timeout+0x1cc/0x310
>> kernel:  ? __wake_up_common+0x74/0x120
>> kernel:  wait_for_completion+0xba/0x140
>> kernel:  ? wake_up_q+0xa0/0xa0
>> kernel:  __ocfs2_cluster_lock.isra.41+0x3b5/0x820 [ocfs2]
>> kernel:  ? ocfs2_inode_lock_full_nested+0x1fc/0x960 [ocfs2]
>> kernel:  ocfs2_inode_lock_full_nested+0x1fc/0x960 [ocfs2]
>> kernel:  ocfs2_init_security_and_acl+0xbe/0x1d0 [ocfs2]
>> kernel:  ocfs2_reflink+0x436/0x4c0 [ocfs2]
>> kernel:  ? ocfs2_reflink_ioctl+0x2ca/0x360 [ocfs2]
>> kernel:  ocfs2_reflink_ioctl+0x2ca/0x360 [ocfs2]
>> kernel:  ocfs2_ioctl+0x25e/0x670 [ocfs2]
>> kernel:  do_vfs_ioctl+0xa0/0x680
>> kernel:  ksys_ioctl+0x70/0x80
>>
>> In fact, the destination directory(.snapshots) inode dlm lock was acquired by ghe-sle15sp2-nd2, next there is bast message from other nodes to ask ghe-sle15sp2-nd2 downconvert lock, but the operation failed, the kernel message is printed like,
>> kernel: (ocfs2dc-AA35DD9,2560,3):ocfs2_downconvert_lock:3660 ERROR: DLM error -16 while calling ocfs2_dlm_lock on resource M0000000000000000046e0200000000
>> kernel: (ocfs2dc-AA35DD9,2560,3):ocfs2_unblock_lock:3904 ERROR: status = -16
>> kernel: (ocfs2dc-AA35DD9,2560,3):ocfs2_process_blocked_lock:4303 ERROR: status = -16
>>
>> Then, the reflink process tries to acquire this directory inode dlm lock, the process is blocked, the dlm lock resource in memory looks like
>>
>>      l_name = "M0000000000000000046e0200000000",
>>      l_ro_holders = 0,
>>      l_ex_holders = 0,
>>      l_level = 5 '\005',
>>      l_requested = 0 '\000',
>>      l_blocking = 5 '\005',
>>      l_type = 0 '\000',
>>      l_action = 0 '\000',
>>      l_unlock_action = 0 '\000',
>>      l_pending_gen = 645948,
>>
>>
>> So far, I do not know what makes dlm lock function failed, it also looks we do not handle this failure case in dlmglue layer, but I always reproduce this hang with my test script, e.g.
>>
>>    loop=1
>>    while ((loop++)) ; do
>>          for i in `seq 1 100`; do
>>            reflink "/mnt/shared/test" "/mnt/shared/.snapshots /test.${loop}.${i}.`date +%m%d%H%M%S`.`hostname`"
>>          done
>>          usleep 500000
>>          rm -f /mnt/shared/.snapshots/testnode1.qcow2.*.`hostname`
>>    done
>>
>> My patch changes multiple acquiring dest directory inode dlm lock during in ocfs2_reflink function, it avoids the hang issue happen again.The code change also can improve reflink performance in this case.
>>
>> Thanks
>> Gang
> 
> 'status = -16' implies DLM_CANCELGRANT.
> Do you use stack user instead of o2cb? If yes, can you try o2cb with
> your reproducer?

I setup o2cb based ocfs2 clusters with sle15sp2 and oracleLinux8u4.
After two day testing with the same script, I did not encounter dlm_lock 
downconvert failure, the hang issue did not happen.
After my patch was applied, there was not any side effect, the reflink 
performance was doubled in the case.

Thanks
Gang

> 
> Thanks,
> Joseph
>