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

[Ming Lei <ming.lei@redhat.com>]

On Thu, Apr 27, 2023 at 07:19:35PM +0800, Baokun Li wrote:
> On 2023/4/27 18:01, Ming Lei wrote:
> > On Thu, Apr 27, 2023 at 02:36:51PM +0800, 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.
> > vfs_write may not find anything wrong, but ext4 remount could see that
> > disk is gone, which might happen during or after remount, however.
> > 
> > > 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
> > > ```
> > Thanks for the info which is really helpful for me to understand the
> > problem.
> > 
> > > > > 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().
> > I have another concern, if 'dd' isn't killed, dirty pages won't be cleaned, and
> > these (big amount)memory becomes not usable, and typical scenario could be USB HDD
> > unplugged.
> > 
> > 
> > thanks,
> > Ming
> Yes, it is unreasonable to continue writing data with the previously opened
> fd after
> the file system becomes read-only, resulting in dirty page accumulation.
> 
> I provided a patch in another reply.
> Could you help test if it can solve your problem?
> If it can indeed solve your problem, I will officially send it to the email
> list.

OK, I will test it tomorrow.

But I am afraid if it can avoid the issue completely because the
old write task hang in balance_dirty_pages() may still write/dirty pages
if it is one very big size write IO.

Thanks,
Ming