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

[Gang He]

Hi Alex,


>>> 
> 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'.
Before we do a direct-io, I need to check if this is a overwrite allocated blocks IO.
If not, we will return  -EAGAIN in 'IOCB_NOWAIT' mode. this should not trigger any block allocation.
I am not sure if we understand your concern totally.

Thanks
Gang 

> 
> 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,
>>>>>>
>>>>
>>>>
>>>> .
>>>>
>> 
>> .
>>