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 22:52:12 +0800
From: Chao Yu <chao@...nel.org>
To: Zhiguo Niu <niuzhiguo84@...il.com>
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

On 2024/6/26 10:01, Zhiguo Niu wrote:
> 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"?

Oops, too rush to send the patch...

> another want to confirm is, "thread B" in commit msg just doing buffer
> read, so this modification just cover direct read case？

Oh, the fix is incorrect, will look into it soon.

Thanks,

> thanks！
>>
>>          return false;
>>   }
>> --
>> 2.40.1
>>
>>
>>
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