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

[Chao Yu]

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.