Re: [ext4 io hang] buffered write io hang in balance_dirty_pages

[Baokun Li <libaokun1@huawei.com>]

On 2023/4/27 12:50, Ming Lei wrote:
> Hello Matthew,
>
> On Thu, Apr 27, 2023 at 04:58:36AM +0100, Matthew Wilcox wrote:
>> On Thu, Apr 27, 2023 at 10:20:28AM +0800, Ming Lei wrote:
>>> Hello Guys,
>>>
>>> I got one report in which buffered write IO hangs in balance_dirty_pages,
>>> after one nvme block device is unplugged physically, then umount can't
>>> succeed.
>> That's a feature, not a bug ... the dd should continue indefinitely?
> Can you explain what the feature is? And not see such 'issue' or 'feature'
> on xfs.
>
> The device has been gone, so IMO it is reasonable to see FS buffered write IO
> failed. Actually dmesg has shown that 'EXT4-fs (nvme0n1): Remounting
> filesystem read-only'. Seems these things may confuse user.


The reason for this difference is that ext4 and xfs handle errors 
differently.

ext4 remounts the filesystem as read-only or even just continues, 
vfs_write does not check for these.

xfs shuts down the filesystem, so it returns a failure at 
xfs_file_write_iter when it finds an error.


``` ext4
ksys_write
  vfs_write
   ext4_file_write_iter
    ext4_buffered_write_iter
     ext4_write_checks
      file_modified
       file_modified_flags
        __file_update_time
         inode_update_time
          generic_update_time
           __mark_inode_dirty
            ext4_dirty_inode ---> 2. void func, No propagating errors out
             __ext4_journal_start_sb
              ext4_journal_check_start ---> 1. Error found, remount-ro
     generic_perform_write ---> 3. No error sensed, continue
      balance_dirty_pages_ratelimited
       balance_dirty_pages_ratelimited_flags
        balance_dirty_pages
         // 4. Sleeping waiting for dirty pages to be freed
         __set_current_state(TASK_KILLABLE)
         io_schedule_timeout(pause);
```

``` xfs
ksys_write
  vfs_write
   xfs_file_write_iter
    if (xfs_is_shutdown(ip->i_mount))
      return -EIO;    ---> dd fail
```
>> balance_dirty_pages() is sleeping in KILLABLE state, so kill -9 of
>> the dd process should succeed.
> Yeah, dd can be killed, however it may be any application(s), :-)
>
> Fortunately it won't cause trouble during reboot/power off, given
> userspace will be killed at that time.
>
>
>
> Thanks,
> Ming
>
Don't worry about that, we always set the current thread to TASK_KILLABLE

while waiting in balance_dirty_pages().


-- 
With Best Regards,
Baokun Li
.