linux-kernel - Re: [Ocfs2-devel] [PATCH 2/3] ocfs2: add ocfs2_overwrite

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Date:   Tue, 28 Nov 2017 00:38:54 -0700
From:   "Gang He" <ghe@...e.com>
To:     <alex.chen@...wei.com>
Cc:     <jlbec@...lplan.org>, <hch@....de>, <ocfs2-devel@....oracle.com>,
        "Goldwyn Rodrigues" <RGoldwyn@...e.com>, <mfasheh@...sity.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [Ocfs2-devel] [PATCH 2/3] ocfs2: add ocfs2_overwrite_io
 function

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@...e.com>
>>>> ---
>>>>  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.

> 
> 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.

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