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

Baokun Li
2023-04-27T06:37:18Z

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
.