Re: [PATCH v2 2/3] ext4:Add two functions splitting an extent.

[Mingming Cao <cmm@us.ibm.com>]

On Mon, 2011-05-02 at 19:05 -0700, Yongqiang Yang wrote:
> v0 -> v1:
>    -- coding style
>    -- try to merge extents in zeroout case too.
> 
> 1] Add a function named ext4_split_extent_at() which splits an extent
>    into two extents at given logical block.
> 
> 2] Add a function called ext4_split_extent() which splits an extent
>    into three extents.
> 
> Signed-off-by: Yongqiang Yang <xiaoqiangnk@gmail.com>
> Tested-by: Allison Henderson <achender@linux.vnet.ibm.com>
> ---
>  fs/ext4/extents.c |  187 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 187 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 11f30d2..db1d67c 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -2554,6 +2554,193 @@ static int ext4_ext_zeroout(struct inode *inode, struct ext4_extent *ex)
>  	return ret;
>  }
> 
> +/*
> + * used by extent splitting.
> + */
> +#define EXT4_EXT_MAY_ZEROOUT	0x1  /* safe to zeroout if split fails \
> +					due to ENOSPC */
> +#define EXT4_EXT_MARK_UNINIT1	0x2  /* mark first half uninitialized */
> +#define EXT4_EXT_MARK_UNINIT2	0x4  /* mark second half uninitialized */
> +
> +/*
> + * ext4_split_extent_at() splits an extent at given block.
> + *
> + * @handle: the journal handle
> + * @inode: the file inode
> + * @path: the path to the extent
> + * @split: the logical block where the extent is splitted.
> + * @split_flags: indicates if the extent could be zeroout if split fails, and
> + *		 the states(init or uninit) of new extents.
> + * @flags: flags used to insert new extent to extent tree.
> + *
> + *
> + * Splits extent [a, b] into two extents [a, @split) and [@split, b], states
> + * of which are deterimined by split_flag.
> + *
> + * There are two cases:
> + *  a> the extent are splitted into two extent.
> + *  b> split is not needed, and just mark the extent.
> + *
> + * return 0 on success.
> + */
> +static int ext4_split_extent_at(handle_t *handle,
> +			     struct inode *inode,
> +			     struct ext4_ext_path *path,
> +			     ext4_lblk_t split,
> +			     int split_flag,
> +			     int flags)
> +{
> +	ext4_fsblk_t newblock;
> +	ext4_lblk_t ee_block;
> +	struct ext4_extent *ex, newex, orig_ex;
> +	struct ext4_extent *ex2 = NULL;
> +	unsigned int ee_len, depth;
> +	int err = 0;
> +
> +	ext_debug("ext4_split_extents_at: inode %lu, logical"
> +		"block %llu\n", inode->i_ino, (unsigned long long)split);
> +
> +	ext4_ext_show_leaf(inode, path);
> +
> +	depth = ext_depth(inode);
> +	ex = path[depth].p_ext;
> +	ee_block = le32_to_cpu(ex->ee_block);
> +	ee_len = ext4_ext_get_actual_len(ex);
> +	newblock = split - ee_block + ext4_ext_pblock(ex);
> +
> +	BUG_ON(split < ee_block || split >= (ee_block + ee_len));
> +
> +	err = ext4_ext_get_access(handle, inode, path + depth);
> +	if (err)
> +		goto out;
> +
> +	if (split == ee_block) {
> +		/*
> +		 * case b: block @split is the block that the extent begins with
> +		 * then we just change the state of the extent, and splitting
> +		 * is not needed.
> +		 */
> +		if (split_flag & EXT4_EXT_MARK_UNINIT2)
> +			ext4_ext_mark_uninitialized(ex);
> +		else
> +			ext4_ext_mark_initialized(ex);
> +
> +		if (!(flags & EXT4_GET_BLOCKS_PRE_IO))
> +			ext4_ext_try_to_merge(inode, path, ex);
> +
> +		err = ext4_ext_dirty(handle, inode, path + depth);
> +		goto out;
> +	}
> +
> +	/* case a */
> +	memcpy(&orig_ex, ex, sizeof(orig_ex));
> +	ex->ee_len = cpu_to_le16(split - ee_block);
> +	if (split_flag & EXT4_EXT_MARK_UNINIT1)
> +		ext4_ext_mark_uninitialized(ex);
> +
> +	/*
> +	 * path may lead to new leaf, not to original leaf any more
> +	 * after ext4_ext_insert_extent() returns,
> +	 */
> +	err = ext4_ext_dirty(handle, inode, path + depth);
> +	if (err)
> +		goto fix_extent_len;
> +
> +	ex2 = &newex;
> +	ex2->ee_block = cpu_to_le32(split);
> +	ex2->ee_len   = cpu_to_le16(ee_len - (split - ee_block));
> +	ext4_ext_store_pblock(ex2, newblock);
> +	if (split_flag & EXT4_EXT_MARK_UNINIT2)
> +		ext4_ext_mark_uninitialized(ex2);
> +
> +	err = ext4_ext_insert_extent(handle, inode, path, &newex, flags);
> +	if (err == -ENOSPC && (EXT4_EXT_MAY_ZEROOUT & split_flag)) {
> +		err = ext4_ext_zeroout(inode, &orig_ex);
> +		if (err)
> +			goto fix_extent_len;
> +		/* update the extent length and mark as initialized */
> +		ex->ee_len = cpu_to_le32(ee_len);
> +		ext4_ext_try_to_merge(inode, path, ex);
> +		err = ext4_ext_dirty(handle, inode, path + depth);
> +		goto out;
> +	} else if (err)
> +		goto fix_extent_len;
> +
> +out:
> +	ext4_ext_show_leaf(inode, path);
> +	return err;
> +
> +fix_extent_len:
> +	ex->ee_len = orig_ex.ee_len;
> +	ext4_ext_dirty(handle, inode, path + depth);
> +	return err;
> +}
> +
> +/*
> + * ext4_split_extents() splits an extent and mark extent which is covered
> + * by @map as split_flags indicates
> + *
> + * It may result in splitting the extent into multiple extents (upto three)
> + * There are three possibilities:
> + *   a> There is no split required
> + *   b> Splits in two extents: Split is happening at either end of the extent
> + *   c> Splits in three extents: Somone is splitting in middle of the extent
> + *
> + */
> +static int ext4_split_extent(handle_t *handle,
> +			      struct inode *inode,
> +			      struct ext4_ext_path *path,
> +			      struct ext4_map_blocks *map,
> +			      int split_flag,
> +			      int flags)
> +{
> +	ext4_lblk_t ee_block;
> +	struct ext4_extent *ex;
> +	unsigned int ee_len, depth;
> +	int err = 0;
> +	int uninitialized;
> +	int split_flag1, flags1;
> +
> +	depth = ext_depth(inode);
> +	ex = path[depth].p_ext;
> +	ee_block = le32_to_cpu(ex->ee_block);
> +	ee_len = ext4_ext_get_actual_len(ex);
> +	uninitialized = ext4_ext_is_uninitialized(ex);
> +
> +	if (map->m_lblk + map->m_len < ee_block + ee_len) {
> +		split_flag1 = split_flag & EXT4_EXT_MAY_ZEROOUT ?
> +			      EXT4_EXT_MAY_ZEROOUT : 0;
> +		flags1 = flags | EXT4_GET_BLOCKS_PRE_IO;
> +		if (uninitialized)
> +			split_flag1 |= EXT4_EXT_MARK_UNINIT1 |
> +				       EXT4_EXT_MARK_UNINIT2;
> +		err = ext4_split_extent_at(handle, inode, path,
> +				map->m_lblk + map->m_len, split_flag1, flags1);
> +	}
> +

Hmm, I could not see the zeroout extent gets marked as initialized here.
Nothing wrong to expose the wrong data, but certainly we are not take
advantage of zero out,  Perhaps I missed something?

It would be nice to add some comments to describe the difference of
split_flag1, flags1, flags:-) Thanks.

Also, I think we miss error handling here. What if the first split
failed and return error here? we still proceed to to do next split? I
think we should go to the err exit, isnt?


> +	ext4_ext_drop_refs(path);
> +	path = ext4_ext_find_extent(inode, map->m_lblk, path);
> +	if (IS_ERR(path))
> +		return PTR_ERR(path);
> +
> +	if (map->m_lblk >= ee_block) {
> +		split_flag1 = split_flag & EXT4_EXT_MAY_ZEROOUT ?
> +			      EXT4_EXT_MAY_ZEROOUT : 0;
> +		if (uninitialized)
> +			split_flag1 |= EXT4_EXT_MARK_UNINIT1;
> +		if (split_flag & EXT4_EXT_MARK_UNINIT2)
> +			split_flag1 |= EXT4_EXT_MARK_UNINIT2;
> +		err = ext4_split_extent_at(handle, inode, path,
> +				map->m_lblk, split_flag1, flags);
> +		if (err)
> +			goto out;
> +	}
> +
> +	ext4_ext_show_leaf(inode, path);
> +out:
> +	return err ? err : map->m_len;
> +}
> +
>  #define EXT4_EXT_ZERO_LEN 7
>  /*
>   * This function is called by ext4_ext_map_blocks() if someone tries to write