Re: [Ocfs2-devel] [PATCH 2/3] ocfs2: add ocfs2_overwrite_io function

[alex chen]

Hi Gang,

On 2017/11/28 15:38, Gang He wrote:
> Hi Alex,
> 
> 
>>>>
>> Hi Gang,
>>
>> On 2017/11/28 13:33, Gang He wrote:
>>> Hello Alex,
>>>
>>>
>>>>>>
>>>> Hi Gang,
>>>>
>>>> On 2017/11/27 17:46, Gang He wrote:
>>>>> Add ocfs2_overwrite_io function, which is used to judge if
>>>>> overwrite allocated blocks, otherwise, the write will bring extra
>>>>> block allocation overhead.
>>>>>
>>>>> Signed-off-by: Gang He <ghe@xxxxxxxx>
>>>>> ---
>>>>>  fs/ocfs2/extent_map.c | 67 
>>>> +++++++++++++++++++++++++++++++++++++++++++++++++++
>>>>>  fs/ocfs2/extent_map.h |  3 +++
>>>>>  2 files changed, 70 insertions(+)
>>>>>
>>>>> diff --git a/fs/ocfs2/extent_map.c b/fs/ocfs2/extent_map.c
>>>>> index e4719e0..98bf325 100644
>>>>> --- a/fs/ocfs2/extent_map.c
>>>>> +++ b/fs/ocfs2/extent_map.c
>>>>> @@ -832,6 +832,73 @@ int ocfs2_fiemap(struct inode *inode, struct 
>>>> fiemap_extent_info *fieinfo,
>>>>>  	return ret;
>>>>>  }
>>>>>  
>>>>> +/* Is IO overwriting allocated blocks? */
>>>>> +int ocfs2_overwrite_io(struct inode *inode, u64 map_start, u64 map_len,
>>>>> +		       int wait)
>>>>> +{
>>>>> +	int ret = 0, is_last;
>>>>> +	u32 mapping_end, cpos;
>>>>> +	struct ocfs2_super *osb = OCFS2_SB(inode->i_sb);
>>>>> +	struct buffer_head *di_bh = NULL;
>>>>> +	struct ocfs2_extent_rec rec;
>>>>> +
>>>>> +	if (wait)
>>>>> +		ret = ocfs2_inode_lock(inode, &di_bh, 0);
>>>>> +	else
>>>>> +		ret = ocfs2_try_inode_lock(inode, &di_bh, 0);
>>>>> +	if (ret)
>>>>> +		goto out;
>>>>> +
>>>>> +	if (wait)
>>>>> +		down_read(&OCFS2_I(inode)->ip_alloc_sem);
>>>>> +	else {
>>>>> +		if (!down_read_trylock(&OCFS2_I(inode)->ip_alloc_sem)) {
>>>>> +			ret = -EAGAIN;
>>>>> +			goto out_unlock1;
>>>>> +		}
>>>>> +	}
>>>>> +
>>>>> +	if ((OCFS2_I(inode)->ip_dyn_features & OCFS2_INLINE_DATA_FL) &&
>>>>> +	   ((map_start + map_len) <= i_size_read(inode)))
>>>>> +		goto out_unlock2;
>>>>> +
>>>>> +	cpos = map_start >> osb->s_clustersize_bits;
>>>>> +	mapping_end = ocfs2_clusters_for_bytes(inode->i_sb,
>>>>> +					       map_start + map_len);
>>>>> +	is_last = 0;
>>>>> +	while (cpos < mapping_end && !is_last) {
>>>>> +		ret = ocfs2_get_clusters_nocache(inode, di_bh, cpos,
>>>>> +						 NULL, &rec, &is_last);
>>>>> +		if (ret) {
>>>>> +			mlog_errno(ret);
>>>>> +			goto out_unlock2;
>>>>> +		}
>>>>> +
>>>>> +		if (rec.e_blkno == 0ULL)
>>>>> +			break;
>>>> I think here the blocks is not overwrite, because the hold is found and the 
>>>> blocks
>>>> should be allocated.
>>> If the rec.e_blkno == NULL, this means there is a hole.
>>> The file hole means that these blocks are not allocated, it does not like 
>> unwritten block.
>>> The unwritten blocks means that these blocks are allocated, but still have 
>> not been unwritten. 
>>>
>> If we break the loop when we find the hold, out of this function we will 
>> allocate the blocks in
>> ocfs2_file_write_iter()->..->ocfs2_direct_IO->__blockdev_direct_IO->..->ocfs2_dio_wr_g
>> et_block()
>> ->ocfs2_write_begin_nolock. Does this violate the semantics of 'IOCB_NOWAIT';
> Yes, then we need to check if this is a overwrite before doing direct-io.
>

I mean here we should return 0 instead of break and we should immediately return -EAGAIN
to upper apps, otherwise, some block allocation will be happen, which violates the
semantics of 'IOCB_NOWAIT'.

Thanks,
Alex

>>
>> BTW, should we consider the down_write() and ocfs2_inode_lock() in 
>> ocfs2_dio_wr_get_block() when
>> the flag 'IOCB_NOWAIT' is set;
> I think that we should not consider that layer lock, otherwise, the code change will become more and more complex and big.
> I also refer to ext4 file system code change for this feature(728fbc0e10b7f3ce2ee043b32e3453fd5201c055), they did not do any change in that layer.
> 

OK.

> Thanks
> Gang
> 
>>
>>>>> +
>>>>> +		if (rec.e_flags & OCFS2_EXT_REFCOUNTED)
>>>>> +			break;
>>>>> +
>>>>> +		cpos = le32_to_cpu(rec.e_cpos) +
>>>>> +			le16_to_cpu(rec.e_leaf_clusters);
>>>>> +	}
>>>>> +
>>>>> +	if (cpos < mapping_end)
>>>>> +		ret = 1;
>>>>> +
>>>>> +out_unlock2:
>>>>
>>>> I think the 'out_up_read' is more readable than the 'out_unlock2' .
>>> Ok, I will use more readable tag here.
>>>>
>>>>> +	brelse(di_bh);
>>>>> +
>>>>> +	up_read(&OCFS2_I(inode)->ip_alloc_sem);
>>>>> +
>>>>> +out_unlock1:
>>>>
>>>> We should release buffer head here.
>>>>
>>>>> +	ocfs2_inode_unlock(inode, 0);
>>>>> +
>>>>> +out:
>>>>> +	return (ret ? 0 : 1);
>>>>> +}
>>>>> +
>>>>>  int ocfs2_seek_data_hole_offset(struct file *file, loff_t *offset, int 
>>>> whence)
>>>>>  {
>>>>>  	struct inode *inode = file->f_mapping->host;
>>>>> diff --git a/fs/ocfs2/extent_map.h b/fs/ocfs2/extent_map.h
>>>>> index 67ea57d..fd9e86a 100644
>>>>> --- a/fs/ocfs2/extent_map.h
>>>>> +++ b/fs/ocfs2/extent_map.h
>>>>> @@ -53,6 +53,9 @@ int ocfs2_extent_map_get_blocks(struct inode *inode, u64 
>>>> v_blkno, u64 *p_blkno,
>>>>>  int ocfs2_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
>>>>>  		 u64 map_start, u64 map_len);
>>>>>  
>>>>> +int ocfs2_overwrite_io(struct inode *inode, u64 map_start, u64 map_len,
>>>>> +		       int wait);
>>>>> +
>>>>>  int ocfs2_seek_data_hole_offset(struct file *file, loff_t *offset, int 
>>>> origin);
>>>>>  
>>>>>  int ocfs2_xattr_get_clusters(struct inode *inode, u32 v_cluster,
>>>>>
>>>
>>>
>>> .
>>>
> 
> .
>