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Date:   Thu, 27 Apr 2023 14:36:51 +0800
From:   Baokun Li <>
To:     Ming Lei <>,
        Matthew Wilcox <>
CC:     Theodore Ts'o <>, <>,
        Andreas Dilger <>,
        Andrew Morton <>,
        <>, <>,
        Dave Chinner <>,
        Eric Sandeen <>,
        Christoph Hellwig <>, Zhang Yi <>,
        yangerkun <>,
        Baokun Li <>
Subject: Re: [ext4 io hang] buffered write io hang in balance_dirty_pages

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 

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
            ext4_dirty_inode ---> 2. void func, No propagating errors out
              ext4_journal_check_start ---> 1. Error found, remount-ro
     generic_perform_write ---> 3. No error sensed, continue
         // 4. Sleeping waiting for dirty pages to be freed

``` xfs
    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

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