Re: [bug report] ext4 misses final i_size meta sync under O_DIRECT | O_SYNC semantics after iomap DIO conversion

[Jan Kara <jack@suse.cz>]

Hello!

On Tue 19-09-23 14:00:04, Gao Xiang wrote:
> Our consumer reports a behavior change between pre-iomap and iomap
> direct io conversion:
> 
> If the system crashes after an appending write to a file open with
> O_DIRECT | O_SYNC flag set, file i_size won't be updated even if
> O_SYNC was marked before.
> 
> It can be reproduced by a test program in the attachment with
> gcc -o repro repro.c && ./repro testfile && echo c > /proc/sysrq-trigger
> 
> After some analysis, we found that before iomap direct I/O conversion,
> the timing was roughly (taking Linux 3.10 codebase as an example):
> 
> 	..
> 	- ext4_file_dio_write
> 	  - __generic_file_aio_write
> 	      ..
> 	    - ext4_direct_IO  # generic_file_direct_write
> 	      - ext4_ext_direct_IO
> 	        - ext4_ind_direct_IO  # final_size > inode->i_size
> 	          - ..
> 	          - ret = blockdev_direct_IO()
> 	          - i_size_write(inode, end) # orphan && ret > 0 &&
> 	                                   # end > inode->i_size
> 	          - ext4_mark_inode_dirty()
> 	          - ...
> 	  - generic_write_sync  # handling O_SYNC
> 
> So the dirty inode meta will be committed into journal immediately
> if O_SYNC is set.  However, After commit 569342dc2485 ("ext4: move
> inode extension/truncate code out from ->iomap_end() callback"),
> the new behavior seems as below:
> 
> 	..
> 	- ext4_dio_write_iter
> 	  - ext4_dio_write_checks  # extend = 1
> 	  - iomap_dio_rw
> 	      - __iomap_dio_rw
> 	      - iomap_dio_complete
> 	        - generic_write_sync
> 	  - ext4_handle_inode_extension  # extend = 1
> 
> So that i_size will be recorded only after generic_write_sync() is
> called.  So O_SYNC won't flush the update i_size to the disk.

Indeed, that looks like a bug. Thanks for report!

> On the other side, after a quick look of XFS side, it will record
> i_size changes in xfs_dio_write_end_io() so it seems that it doesn't
> have this problem.

Yes, I'm a bit hazy on the details but I think we've decided to call
ext4_handle_inode_extension() directly from ext4_dio_write_iter() because
from ext4_dio_write_end_io() it was difficult to test in a race-free way
whether extending i_size (and i_disksize) is needed or not (we don't
necessarily hold i_rwsem there). I'll think how we could fix the problem
you've reported.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR