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

[Heming Zhao]

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;

- Heming