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Message-ID: <efae597c-d334-498b-9050-1a21bf40e21d@kernel.org>
Date: Wed, 15 May 2024 14:38:39 +0800
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MIME-Version: 1.0
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Subject: Re: [PATCH v2] f2fs: fix to avoid racing in between read and OPU dio
 write
To: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: linux-f2fs-devel@lists.sourceforge.net, linux-kernel@vger.kernel.org
References: <20240510023906.281700-1-chao@kernel.org>
 <ZkOMwKAcKmEPQ4Xz@google.com>
 <fc0d8b1f-0c54-4447-8ceb-3722645f71c2@kernel.org>
 <ZkQ9Uo5713Xpr2n7@google.com>
Content-Language: en-US
From: Chao Yu <chao@kernel.org>
In-Reply-To: <ZkQ9Uo5713Xpr2n7@google.com>
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On 2024/5/15 12:42, Jaegeuk Kim wrote:
> On 05/15, Chao Yu wrote:
>> On 2024/5/15 0:09, Jaegeuk Kim wrote:
>>> On 05/10, 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
>>>>
>>>> This patch introduces a new per-inode i_opu_rwsem lock to avoid
>>>> such race condition.
>>>
>>> Wasn't this supposed to be managed by user-land?
>>
>> Actually, the test case is:
>>
>> 1. mount w/ lfs mode
>> 2. touch file;
>> 3. initialize file w/ 4k zeroed data; fsync;
>> 4. continue triggering dio write 4k zeroed data to file;
>> 5. and meanwhile, continue triggering buf/dio 4k read in file,
>> use md5sum to verify the 4k data;
>>
>> It expects data is all zero, however it turned out it's not.
> 
> Can we check outstanding write bios instead of abusing locks?

I didn't figure out a way to solve this w/o lock, due to:
- write bios can be issued after outstanding write bios check condition,
result in the race.
- once read() detects that there are outstanding write bios, we need to
delay read flow rather than fail it, right? It looks using a lock is more
proper here?

Any suggestion?

Thanks,

> 
>>
>> Thanks,
>>
>>>
>>>>
>>>> Fixes: f847c699cff3 ("f2fs: allow out-place-update for direct IO in LFS mode")
>>>> Signed-off-by: Chao Yu <chao@kernel.org>
>>>> ---
>>>> v2:
>>>> - fix to cover dio read path w/ i_opu_rwsem as well.
>>>>    fs/f2fs/f2fs.h  |  1 +
>>>>    fs/f2fs/file.c  | 28 ++++++++++++++++++++++++++--
>>>>    fs/f2fs/super.c |  1 +
>>>>    3 files changed, 28 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
>>>> index 30058e16a5d0..91cf4b3d6bc6 100644
>>>> --- a/fs/f2fs/f2fs.h
>>>> +++ b/fs/f2fs/f2fs.h
>>>> @@ -847,6 +847,7 @@ struct f2fs_inode_info {
>>>>         /* avoid racing between foreground op and gc */
>>>>         struct f2fs_rwsem i_gc_rwsem[2];
>>>>         struct f2fs_rwsem i_xattr_sem; /* avoid racing between reading and changing EAs */
>>>> +     struct f2fs_rwsem i_opu_rwsem;  /* avoid racing between buf read and opu dio write */
>>>>
>>>>         int i_extra_isize;              /* size of extra space located in i_addr */
>>>>         kprojid_t i_projid;             /* id for project quota */
>>>> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
>>>> index 72ce1a522fb2..4ec260af321f 100644
>>>> --- a/fs/f2fs/file.c
>>>> +++ b/fs/f2fs/file.c
>>>> @@ -4445,6 +4445,7 @@ static ssize_t f2fs_dio_read_iter(struct kiocb *iocb, struct iov_iter *to)
>>>>         const loff_t pos = iocb->ki_pos;
>>>>         const size_t count = iov_iter_count(to);
>>>>         struct iomap_dio *dio;
>>>> +     bool do_opu = f2fs_lfs_mode(sbi);
>>>>         ssize_t ret;
>>>>
>>>>         if (count == 0)
>>>> @@ -4457,8 +4458,14 @@ static ssize_t f2fs_dio_read_iter(struct kiocb *iocb, struct iov_iter *to)
>>>>                         ret = -EAGAIN;
>>>>                         goto out;
>>>>                 }
>>>> +             if (do_opu && !f2fs_down_read_trylock(&fi->i_opu_rwsem)) {
>>>> +                     f2fs_up_read(&fi->i_gc_rwsem[READ]);
>>>> +                     ret = -EAGAIN;
>>>> +                     goto out;
>>>> +             }
>>>>         } else {
>>>>                 f2fs_down_read(&fi->i_gc_rwsem[READ]);
>>>> +             f2fs_down_read(&fi->i_opu_rwsem);
>>>>         }
>>>>
>>>>         /*
>>>> @@ -4477,6 +4484,7 @@ static ssize_t f2fs_dio_read_iter(struct kiocb *iocb, struct iov_iter *to)
>>>>                 ret = iomap_dio_complete(dio);
>>>>         }
>>>>
>>>> +     f2fs_up_read(&fi->i_opu_rwsem);
>>>>         f2fs_up_read(&fi->i_gc_rwsem[READ]);
>>>>
>>>>         file_accessed(file);
>>>> @@ -4523,7 +4531,13 @@ static ssize_t f2fs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
>>>>         if (f2fs_should_use_dio(inode, iocb, to)) {
>>>>                 ret = f2fs_dio_read_iter(iocb, to);
>>>>         } else {
>>>> +             bool do_opu = f2fs_lfs_mode(F2FS_I_SB(inode));
>>>> +
>>>> +             if (do_opu)
>>>> +                     f2fs_down_read(&F2FS_I(inode)->i_opu_rwsem);
>>>>                 ret = filemap_read(iocb, to, 0);
>>>> +             if (do_opu)
>>>> +                     f2fs_up_read(&F2FS_I(inode)->i_opu_rwsem);
>>>>                 if (ret > 0)
>>>>                         f2fs_update_iostat(F2FS_I_SB(inode), inode,
>>>>                                                 APP_BUFFERED_READ_IO, ret);
>>>> @@ -4748,14 +4762,22 @@ static ssize_t f2fs_dio_write_iter(struct kiocb *iocb, struct iov_iter *from,
>>>>                         ret = -EAGAIN;
>>>>                         goto out;
>>>>                 }
>>>> +             if (do_opu && !f2fs_down_write_trylock(&fi->i_opu_rwsem)) {
>>>> +                     f2fs_up_read(&fi->i_gc_rwsem[READ]);
>>>> +                     f2fs_up_read(&fi->i_gc_rwsem[WRITE]);
>>>> +                     ret = -EAGAIN;
>>>> +                     goto out;
>>>> +             }
>>>>         } else {
>>>>                 ret = f2fs_convert_inline_inode(inode);
>>>>                 if (ret)
>>>>                         goto out;
>>>>
>>>>                 f2fs_down_read(&fi->i_gc_rwsem[WRITE]);
>>>> -             if (do_opu)
>>>> +             if (do_opu) {
>>>>                         f2fs_down_read(&fi->i_gc_rwsem[READ]);
>>>> +                     f2fs_down_write(&fi->i_opu_rwsem);
>>>> +             }
>>>>         }
>>>>
>>>>         /*
>>>> @@ -4779,8 +4801,10 @@ static ssize_t f2fs_dio_write_iter(struct kiocb *iocb, struct iov_iter *from,
>>>>                 ret = iomap_dio_complete(dio);
>>>>         }
>>>>
>>>> -     if (do_opu)
>>>> +     if (do_opu) {
>>>> +             f2fs_up_write(&fi->i_opu_rwsem);
>>>>                 f2fs_up_read(&fi->i_gc_rwsem[READ]);
>>>> +     }
>>>>         f2fs_up_read(&fi->i_gc_rwsem[WRITE]);
>>>>
>>>>         if (ret < 0)
>>>> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
>>>> index daf2c4dbe150..b4ed3b094366 100644
>>>> --- a/fs/f2fs/super.c
>>>> +++ b/fs/f2fs/super.c
>>>> @@ -1428,6 +1428,7 @@ static struct inode *f2fs_alloc_inode(struct super_block *sb)
>>>>         init_f2fs_rwsem(&fi->i_gc_rwsem[READ]);
>>>>         init_f2fs_rwsem(&fi->i_gc_rwsem[WRITE]);
>>>>         init_f2fs_rwsem(&fi->i_xattr_sem);
>>>> +     init_f2fs_rwsem(&fi->i_opu_rwsem);
>>>>
>>>>         /* Will be used by directory only */
>>>>         fi->i_dir_level = F2FS_SB(sb)->dir_level;
>>>> --
>>>> 2.40.1