[Ocfs2-devel] report BUG: io_uring triggers umount error

[Joseph Qi]

On 2/26/23 11:22 PM, Heming Zhao wrote:
> On Sun, Feb 26, 2023 at 04:45:44PM +0800, Heming Zhao via Ocfs2-devel wrote:
>> On Fri, Feb 24, 2023 at 06:59:47PM +0800, Joseph Qi wrote:
>>>
>>>
>>> On 2/24/23 4:03 PM, Heming Zhao wrote:
>>>> On 2/24/23 3:52 PM, Joseph Qi wrote:
>>>>>
>>>>>
>>>>> On 2/24/23 3:48 PM, Heming Zhao via Ocfs2-devel wrote:
>>>>>> On 2/24/23 2:54 PM, Joseph Qi wrote:
>>>>>>> I can reproduce this in my local VM.
>>>>>>> I've traced ocfs2_dismount_volume and found that it hasn't been called.
>>>>>>> So EBUSY is returned in VFS layer. I guess something wrong when doing
>>>>>>> a copy with linked SQEs (normal copy seems no problem).
>>>>>>>
>>>>>>
>>>>>> I am inclined to agree with you. I also test liburing examples apps
>>>>>> on ext4 partition, everything looks fine.
>>>>>>
>>>>>> I used below bpftrace method, the retval is '3'.
>>>>>>   bpftrace -e 'kr:mnt_get_count{printf("%d\n", retval);}'
>>>>>>
>>>>>> It responds to flow: path_umount() => do_umount => mnt_get_count (gets '3')
>>>>>>
>>>>> Yes, that's the place return EBUSY.
>>>>> So the problem seems to be getmnt/putmnt not match in this case.
>>>>>
>>>>
>>>> I didn't familiar with setting up kernel bi-search env. I used one last year
>>>> openSUSE tumblweed (with kernel 5.16.2), this umount issue doesn't exist.
>>>> So there is a possibility one ocfs2 commit introduced this issue.
>>> You can checkout each mailine version like Linux 6.0, 6.1, ... and try
>>> to check if it can be reproduced.
>>
>> I drop this method, my machine cpu is old, compiling kernel will take too much
>> time.
>>
>>>
>>> I've tried trace mntget/mntput using the following bpftrace script,
>>> link-cp output shows it misses a fput.
>>>
>>> #include <linux/mount.h>
>>> #include <linux/string.h>
>>>
>>> kprobe:mntget
>>> {
>>> 	$n = ((struct vfsmount *)arg0)->mnt_sb->s_type->name;
>>>
>>> 	if (!strncmp(str($n), "ocfs2", 5)) {
>>> 		@[comm] += 1;
>>>
>>> 		printf("%s", kstack);
>>> 	}
>>> }
>>>
>>> kprobe:mntput
>>> {
>>> 	$n = ((struct vfsmount *)arg0)->mnt_sb->s_type->name;
>>>
>>> 	if (!strncmp(str($n), "ocfs2", 5)) {
>>> 		@[comm] +=1;
>>>
>>> 		printf("%s", kstack);
>>> 	}
>>> }
>>>
>>
>> I used below script to find source file hold f_count.
>>
>> e.g: ./link-cp /mnt/aa.bin /mnt/1
>> The script will show f_count of aa.bin only down to 3. We expect f_count value
>> down to 1 then trigger mntput.
>>
>> ```
>> #include <linux/mount.h>
>> #include <linux/string.h>
>> #include <linux/dcache.h>
>> #include <linux/path.h>
>> #include <linux/fs_struct.h>
>> #include <linux/fs.h>
>> #include <linux/io_uring_types.h>
>>
>> k:fput
>> {
>> 	$f = (struct file *)arg0;
>> 	$cnt = $f->f_count;
>>
>> 	if (!strncmp(comm, "link-cp", 7)) {
>> 		printf("\n [link-cp] fput() name:%s f_count:%lld <%p> mnt:%p\n",
>> 			   	str($f->f_path.dentry->d_name.name), $cnt.counter, kptr($f), kptr($f->f_path.mnt));
>> 		//printf("%s\n", kstack);
>> 	}
>> }
>> ```
>>
> 
> I am not familiar with io_uring, and can't make sure my analysis is correct.
> Because ocfs2_file_read_iter() returns -EOPNOTSUPP, then fails
> io_iter_do_read(), the io_issue_sqe missing fput() which causes umount failure
> issue.
> 
> io_issue_sqe
>  + io_assign_file //call fget(), but missing fput()
>  + def->issue()
>  |  io_read
>  |   + io_iter_do_read
>  |   |  ocfs2_file_read_iter
>  |   |   return: -EOPNOTSUPP //iocb->ki_flags:0x8
>  |   + kiocb_done
>  |      + io_rw_done(&rw->kiocb, ret)
>  |      |   kiocb->ki_complete(kiocb, ret)
>  |      |    io_complete_rw
>  |      |      __io_complete_rw_common
>  |      |       + req->flags |= REQ_F_REISSUE | REQ_F_PARTIAL_IO;
>  |      |       + return true
>  |      + io_req_task_queue_reissue/io_req_task_queue_fail
>  |      + return IOU_ISSUE_SKIP_COMPLETE;
>  |
>  + return 0;
> 
Seems not the case.
Reissue will queue work to iowq, and fput will finally get called in
io_wq_free_work() once work is handled.

Thanks,
Joseph