Re: Dwrite with non-aligned offset and size

[He YunLei <heyunlei@huawei.com>]

On 2015/7/3 16:02, Chao Yu wrote:
> Hi Yunlei,
>
> Sorry for the long delay.
>
>> -----Original Message-----
>> From: He YunLei [mailto:heyunlei@huawei.com]
>> Sent: Tuesday, June 02, 2015 12:21 PM
>> To: Jaegeuk Kim
>> Cc: linux-fsdevel@vger.kernel.org; linux-f2fs-devel@lists.sourceforge.net
>> Subject: Re: [f2fs-dev] Dwrite with non-aligned offset and size
>>
>> On 2015/6/2 7:01, Jaegeuk Kim wrote:
>>> On Mon, Jun 01, 2015 at 07:55:08PM +0800, He YunLei wrote:
>>>> Hi Jaegeuk,
>>>>
>>>> We run ltp testcase with f2fs and obtain a TFAIL in diotest4, the result in detail is
>>>> as fallow:
>>>>
>>>> dio04
>>>>
>>>> <<<test_start>>>
>>>> tag=dio04 stime=1432278894
>>>> cmdline="diotest4"
>>>> contacts=""
>>>> analysis=exit
>>>> <<<test_output>>>
>>>> diotest4    1  TPASS  :  Negative Offset
>>>> diotest4    2  TPASS  :  removed
>>>> diotest4    3  TFAIL  :  diotest4.c:129: write allows odd count.returns 1: Success
>>>> diotest4    4  TFAIL  :  diotest4.c:183: Odd count of read and write
>>>> diotest4    5  TPASS  :  Read beyond the file size
>>>> ......
>>>>
>>>> the result of ext4 with same environment:
>>>>
>>>> dio04
>>>>
>>>> <<<test_start>>>
>>>> tag=dio04 stime=1432259643
>>>> cmdline="diotest4"
>>>> contacts=""
>>>> analysis=exit
>>>> <<<test_output>>>
>>>> diotest4    1  TPASS  :  Negative Offset
>>>> diotest4    2  TPASS  :  removed
>>>> diotest4    3  TPASS  :  Odd count of read and write
>>>> diotest4    4  TPASS  :  Read beyond the file size
>>>> ......
>>>>
>>>> Does f2fs allow dwrite with non-aligned offset and size? I check the code and found
>>>> dwrite with non-aligned offset and size will turn into buffered write. Whether it will
>>>> have some impact on user layer applications?
>>>
>>> It's not a big deal to return -EINVAL.
>>> When I take a look at other filesystem behaviors, it seems there is no restriction.
>>>
>>
>> Ext4 do a check in the function do_blockdev_direct_IO:
>>
>>            if (align & blocksize_mask) {
>>                if (bdev)
>>                    blkbits = blksize_bits(bdev_logical_block_size(bdev));
>>                blocksize_mask = (1 << blkbits) - 1;
>>                if (align & blocksize_mask)
>>                    goto out;
>>            }
>>
>> It will return -EINVAL if the alignment is not satisfied.
>
> I think we can get hint from the error case description in write(2) manual:
>
> "EINVAL  fd is attached to an object which is unsuitable for writing; or the file
> was  opened with the O_DIRECT flag, and either the address specified in buf, the
> value specified in count, or the current file offset is not suitably aligned."
>
> So if the alignment is not satisfied, we should return '-EINVAL' instead of
> letting user fall back to buffered write.
>
> Do you have time to make and send us a patch for fixing this issue?
>
> Thanks,
>
In man page of open(2)
	
The O_DIRECT flag may impose alignment restrictions on the length and
address of user-space buffers and the file offset of I/Os.  In Linux
alignment restrictions vary by filesystem and kernel version and
might be absent entirely.  However there is currently no
filesystem-independent interface for an application to discover these
restrictions for a given file or filesystem.

So now I don't know which one is better, falling back to buffered write or not?
>>
>> In f2fs, it do the check by check_direct_IO() before blockdev_direct_IO().
>> The difference between the two methods is whether turn dwrite with non-aligned
>> offset and size into buffered write. I am not very clear which one is better!
>>
>> Thanks,
>> He
>>
>>>>
>>>> I wrote a patch, not well tested, how do you think of it?
>>>
>>> Returning the error number would be good to me.
>>> Could you write and sumbit a complete one?
>>>
>>> Thanks,
>>>
>>>>
>>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>>>> index 9bedfa8..ba5d94c 100644
>>>> --- a/fs/f2fs/data.c
>>>> +++ b/fs/f2fs/data.c
>>>> @@ -2010,8 +2010,9 @@ static ssize_t f2fs_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
>>>>           if (f2fs_encrypted_inode(inode) && S_ISREG(inode->i_mode))
>>>>                   return 0;
>>>>
>>>> -       if (check_direct_IO(inode, iter, offset))
>>>> -               return 0;
>>>> +       err = check_direct_IO(inode, iter, offset)
>>>> +       if (err)
>>>> +               return -EINVAL;
>>>>
>>>>           trace_f2fs_direct_IO_enter(inode, offset, count, iov_iter_rw(iter));
>>>>
>>>> I wish you and other developers in this list could help me in a correct way.
>>>>
>>>> Thanks,
>>>> He
>>>
>>> .
>>>
>>
>>
>> ------------------------------------------------------------------------------
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>> Linux-f2fs-devel@lists.sourceforge.net
>> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
>
>
> .
>


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