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

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

Jan Kara <jack@suse.cz> writes:

> On Wed 20-09-23 17:08:19, Ritesh Harjani wrote:
>> Jan Kara <jack@suse.cz> writes:
>> 
>> > 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
>> 
>> Yes, since ext4_handle_inode_extension() will handle inode i_disksize
>> update and mark the inode dirty, generic_write_sync() call should happen
>> after that.
>> 
>> That also means then we don't have any generic FS testcase which can
>> validate this behaviour. 
>
> Yeah.
>

Ok. Let me then first send a fstest in response to this integrity
problem with directio and o_sync.

-ritesh