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
