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

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Date: Wed, 26 Jun 2024 10:01:53 +0800
From: Zhiguo Niu <niuzhiguo84@...il.com>
To: Chao Yu <chao@...nel.org>
Cc: jaegeuk@...nel.org, linux-kernel@...r.kernel.org, 
	linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH v2] f2fs: fix to avoid racing in between read
 and OPU dio write

Chao Yu <chao@...nel.org> 于2024年6月25日周二 22:29写道：
>
> 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
>
> In LFS mode, if there is inflight dio, let's force read to buffered
> IO, this policy won't cover all race cases, however it is a tradeoff
> which avoids abusing lock around IO paths.
>
> Fixes: f847c699cff3 ("f2fs: allow out-place-update for direct IO in LFS mode")
> Signed-off-by: Chao Yu <chao@...nel.org>
> ---
>  fs/f2fs/file.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> index 278573974db4..866f1a34e92b 100644
> --- a/fs/f2fs/file.c
> +++ b/fs/f2fs/file.c
> @@ -882,6 +882,10 @@ static bool f2fs_force_buffered_io(struct inode *inode, int rw)
>                 return true;
>         if (is_sbi_flag_set(sbi, SBI_CP_DISABLED))
>                 return true;
> +       /* In LFS mode, if there is inflight dio, force read to buffered IO */
> +       if (rw == READ && f2fs_lfs_mode(sbi) &&
> +                       atomic_read(&inode->i_dio_count))
> +               return false;
Hi Chao,
A little doubt:)，force “buffered IO” should return "true"?
another want to confirm is, "thread B" in commit msg just doing buffer
read, so this modification just cover direct read case？
thanks！
>
>         return false;
>  }
> --
> 2.40.1
>
>
>
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