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

[Changwei Ge]

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.

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

Just for append-dio

>> 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)
>>
>