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

[Baokun Li <libaokun1@huawei.com>]

On 2023/4/27 14:36, Baokun Li wrote:
> 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().
>
>
On second thought, we can determine if the file system has become read-only
when the ext4_file_write_iter() is called on a write file, even though 
the fs was
not read-only when the file was opened.

This would end the write process early and free up resources like xfs does.
The patch is below, does anyone have any other thoughts?


diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index d101b3b0c7da..d2966268ee41 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -699,6 +699,8 @@ ext4_file_write_iter(struct kiocb *iocb, struct 
iov_iter *from)

         if (unlikely(ext4_forced_shutdown(EXT4_SB(inode->i_sb))))
                 return -EIO;
+       if (unlikely(sb_rdonly(inode->i_sb)))
+               return -EROFS;

  #ifdef CONFIG_FS_DAX
         if (IS_DAX(inode))


-- 
With Best Regards,
Baokun Li
.