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

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Date:   Mon, 27 Nov 2017 22:40:26 -0700
From:   "Gang He" <ghe@...e.com>
To:     <jlbec@...lplan.org>, <ge.changwei@....com>, <hch@....de>,
        "Goldwyn Rodrigues" <RGoldwyn@...e.com>, <mfasheh@...sity.com>
Cc:     <ocfs2-devel@....oracle.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [Ocfs2-devel] [PATCH 2/3] ocfs2: add ocfs2_overwrite_io
 function

Hi Changwei,


>>> 
> Hi,
> Gang
> 
> On 2017/11/27 17:48, 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.
>> 
> 
> Can you elaborate how this overhead is introduced?
> Forgive me, I don't figure it.
If the blocks have been allocated, we just write these block directly.
If these blocks have not been allocated, that means we need to allocate these block firstly before write,
this allocation will bring the IO invoking be blocked, if the upper application does not want take this kind of overhead,
he can pass a nowait flag to avoid and return immediately with a -EAGAIN error.

Thanks
Gang

> 
> Thanks,
> Changwei
> 
>> 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;
>> +
>> +		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:
>> +	brelse(di_bh);
>> +
>> +	up_read(&OCFS2_I(inode)->ip_alloc_sem);
>> +
>> +out_unlock1:
>> +	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,
>>