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
