[Ocfs2-devel] [PATCH] ocfs2: fall back to buffer IO when append dio is disabled with file hole existing

[piaojun]

Hi Changwei,

On 2017/12/19 11:05, Changwei Ge wrote:
> Hi Jun,
> 
> On 2017/12/19 9:48, piaojun wrote:
>> Hi Changwei,
>>
>> On 2017/12/18 20:06, Changwei Ge wrote:
>>> Before ocfs2 supporting allocating clusters while doing append-dio, all append
>>> dio will fall back to buffer io to allocate clusters firstly. Also, when it
>>> steps on a file hole, it will fall back to buffer io, too. But for current
>>> code, writing to file hole will leverage dio to allocate clusters. This is not
>>> right, since whether append-io is enabled tells the capability whether ocfs2 can
>>> allocate space while doing dio.
>>> So introduce file hole check function back into ocfs2.
>>> Once ocfs2 is doing dio upon a file hole with append-dio disabled, it will fall
>>> back to buffer IO to allocate clusters.
>>>
>> 1. Do you mean that filling hole can't go with dio when append-dio is disabled?
> 
> Yes, direct IO will fall back to buffer IO with _append-dio_ disabled.

Why does dio need fall back to buffer-io when append-dio disabled?
Could 'common-dio' on file hole go through direct io process? If not,
could you please point out the necessity.

> 
>> 2. Is your checking-hole just for 'append-dio' or for 'all-common-dio'?
> 
> Just for append-dio
> 

If your patch is just for 'append-dio', I wonder that will have impact
on 'common-dio'.

thanks,
Jun

>>> Signed-off-by: Changwei Ge <ge.changwei at h3c.com>
>>> ---
>>>    fs/ocfs2/aops.c | 44 ++++++++++++++++++++++++++++++++++++++++++--
>>>    1 file changed, 42 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/fs/ocfs2/aops.c b/fs/ocfs2/aops.c
>>> index d151632..a982cf6 100644
>>> --- a/fs/ocfs2/aops.c
>>> +++ b/fs/ocfs2/aops.c
>>> @@ -2414,6 +2414,44 @@ static int ocfs2_dio_end_io(struct kiocb *iocb,
>>>    	return ret;
>>>    }
>>>    
>>> +/*
>>> + * Will look for holes and unwritten extents in the range starting at
>>> + * pos for count bytes (inclusive).
>>> + */
>>> +static int ocfs2_check_range_for_holes(struct inode *inode, loff_t pos,
>>> +				       size_t count)
>>> +{
>>> +	int ret = 0;
>>> +	unsigned int extent_flags;
>>> +	u32 cpos, clusters, extent_len, phys_cpos;
>>> +	struct super_block *sb = inode->i_sb;
>>> +
>>> +	cpos = pos >> OCFS2_SB(sb)->s_clustersize_bits;
>>> +	clusters = ocfs2_clusters_for_bytes(sb, pos + count) - cpos;
>>> +
>>> +	while (clusters) {
>>> +		ret = ocfs2_get_clusters(inode, cpos, &phys_cpos, &extent_len,
>>> +					 &extent_flags);
>>> +		if (ret < 0) {
>>> +			mlog_errno(ret);
>>> +			goto out;
>>> +		}
>>> +
>>> +		if (phys_cpos == 0 || (extent_flags & OCFS2_EXT_UNWRITTEN)) {
>>> +			ret = 1;
>>> +			break;
>>> +		}
>>> +
>>> +		if (extent_len > clusters)
>>> +			extent_len = clusters;
>>> +
>>> +		clusters -= extent_len;
>>> +		cpos += extent_len;
>>> +	}
>>> +out:
>>> +	return ret;
>>> +}
>>> +
>>>    static ssize_t ocfs2_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
>>>    {
>>>    	struct file *file = iocb->ki_filp;
>>> @@ -2429,8 +2467,10 @@ static ssize_t ocfs2_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
>>>    		return 0;
>>>    
>>>    	/* Fallback to buffered I/O if we do not support append dio. */
>>> -	if (iocb->ki_pos + iter->count > i_size_read(inode) &&
>>> -	    !ocfs2_supports_append_dio(osb))
>>> +	if (!ocfs2_supports_append_dio(osb) &&
>>> +			(iocb->ki_pos + iter->count > i_size_read(inode) ||
>>> +			 ocfs2_check_range_for_holes(inode, iocb->ki_pos,
>>> +						     iter->count)))
>> we should check error here, right?
> 
> Accept this point.
> 
> Thanks,
> Changwei
> 
>>
>> thanks,
>> Jun
>>>    		return 0;
>>>    
>>>    	if (iov_iter_rw(iter) == READ)
>>>
>>
> 
> .
>