Re: [f2fs-dev] [PATCH v2] f2fs: fix to avoid racing in between read and OPU dio write

[Chao Yu <chao@kernel.org>]

On 2024/5/15 16:32, Wu Bo wrote:
> On Fri, May 10, 2024 at 10:39:06AM +0800, Chao Yu wrote:
>> If lfs mode is on, buffered read may race w/ OPU dio write as below,
>> it may cause buffered read hits unwritten data unexpectly, and for
>> dio read, the race condition exists as well.
>>
>> Thread A                      Thread B
>> - f2fs_file_write_iter
>>   - f2fs_dio_write_iter
>>    - __iomap_dio_rw
>>     - f2fs_iomap_begin
>>      - f2fs_map_blocks
>>       - __allocate_data_block
>>        - allocated blkaddr #x
>>         - iomap_dio_submit_bio
>>                                - f2fs_file_read_iter
>>                                 - filemap_read
>>                                  - f2fs_read_data_folio
>>                                   - f2fs_mpage_readpages
>>                                    - f2fs_map_blocks
>>                                     : get blkaddr #x
>>                                    - f2fs_submit_read_bio
>>                                IRQ
>>                                - f2fs_read_end_io
>>                                 : read IO on blkaddr #x complete
>> IRQ
>> - iomap_dio_bio_end_io
>>   : direct write IO on blkaddr #x complete
>>
> Looks like every COW filesystem would meet this situation. What's the solution
> of other FS?

I missed to reply this...

Other cow filesystem like btrfs, it will update metadata after data IO completion,
so it is safe.

Thanks,

>> This patch introduces a new per-inode i_opu_rwsem lock to avoid
>> such race condition.
>>


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