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

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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
.



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