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SubjectRe: [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@suse.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,
>>>>
>>
>>
>> .
>>

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