Re: [PATCH v2 8/8] ext4: Allow zeroout when doing written to unwritten split

[Zhang Yi <yizhang089@gmail.com>]

On 1/14/2026 10:57 PM, Ojaswin Mujoo wrote:
> Currently, when we are doing an extent split and convert operation of
> written to unwritten extent (example, as done by ZERO_RANGE), we don't
> allow the zeroout fallback in case the extent tree manipulation fails.
> This is mostly because zeroout might take unsually long and the fact that
> this code path is more tolerant to failures than endio.
> 
> Since we have zeroout machinery in place, we might as well use it hence
> lift this restriction. To mitigate zeroout taking too long respect the
> max zeroout limit here so that the operation finishes relatively fast.
> 
> Also, add kunit tests for this case.
> 
> Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
> Reviewed-by: Jan Kara <jack@suse.cz>

It looks good to me.

Reviewed-by: Zhang Yi <yi.zhang@huawei.com>

> ---
>   fs/ext4/extents-test.c | 71 ++++++++++++++++++++++++++++++++++++++++++
>   fs/ext4/extents.c      | 33 +++++++++++++++-----
>   2 files changed, 96 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/ext4/extents-test.c b/fs/ext4/extents-test.c
> index 86fcac66be6f..d3a26cc8a9ad 100644
> --- a/fs/ext4/extents-test.c
> +++ b/fs/ext4/extents-test.c
> @@ -578,6 +578,41 @@ static const struct kunit_ext_test_param test_split_convert_params[] = {
>   			      { .exp_char = 'X', .off_blk = 1, .len_blk = 1 },
>   			      { .exp_char = 0, .off_blk = 2, .len_blk = 1 } } },
>   
> +	/* writ to unwrit splits */
> +	{ .desc = "split writ extent to 2 extents and convert 1st half unwrit (zeroout)",
> +	  .type = TEST_SPLIT_CONVERT,
> +	  .is_unwrit_at_start = 0,
> +	  .split_flags = EXT4_GET_BLOCKS_CONVERT_UNWRITTEN,
> +	  .split_map = { .m_lblk = 10, .m_len = 1 },
> +	  .nr_exp_ext = 1,
> +	  .exp_ext_state = { { .ex_lblk = 10, .ex_len = 3, .is_unwrit = 0 } },
> +	  .is_zeroout_test = 1,
> +	  .nr_exp_data_segs = 2,
> +	  .exp_data_state = { { .exp_char = 0, .off_blk = 0, .len_blk = 1 },
> +			      { .exp_char = 'X', .off_blk = 1, .len_blk = 2 }}},
> +	{ .desc = "split writ extent to 2 extents and convert 2nd half unwrit (zeroout)",
> +	  .type = TEST_SPLIT_CONVERT,
> +	  .is_unwrit_at_start = 0,
> +	  .split_flags = EXT4_GET_BLOCKS_CONVERT_UNWRITTEN,
> +	  .split_map = { .m_lblk = 11, .m_len = 2 },
> +	  .nr_exp_ext = 1,
> +	  .exp_ext_state = { { .ex_lblk = 10, .ex_len = 3, .is_unwrit = 0 } },
> +	  .is_zeroout_test = 1,
> +	  .nr_exp_data_segs = 2,
> +	  .exp_data_state = { { .exp_char = 'X', .off_blk = 0, .len_blk = 1 },
> +			      { .exp_char = 0, .off_blk = 1, .len_blk = 2 } } },
> +	{ .desc = "split writ extent to 3 extents and convert 2nd half unwrit (zeroout)",
> +	  .type = TEST_SPLIT_CONVERT,
> +	  .is_unwrit_at_start = 0,
> +	  .split_flags = EXT4_GET_BLOCKS_CONVERT_UNWRITTEN,
> +	  .split_map = { .m_lblk = 11, .m_len = 1 },
> +	  .nr_exp_ext = 1,
> +	  .exp_ext_state = { { .ex_lblk = 10, .ex_len = 3, .is_unwrit = 0 } },
> +	  .is_zeroout_test = 1,
> +	  .nr_exp_data_segs = 3,
> +	  .exp_data_state = { { .exp_char = 'X', .off_blk = 0, .len_blk = 1 },
> +			      { .exp_char = 0, .off_blk = 1, .len_blk = 1 },
> +			      { .exp_char = 'X', .off_blk = 2, .len_blk = 1 }}},
>   };
>   
>   static const struct kunit_ext_test_param test_convert_initialized_params[] = {
> @@ -610,6 +645,42 @@ static const struct kunit_ext_test_param test_convert_initialized_params[] = {
>   			     { .ex_lblk = 11, .ex_len = 1, .is_unwrit = 1 },
>   			     { .ex_lblk = 12, .ex_len = 1, .is_unwrit = 0 } },
>   	  .is_zeroout_test = 0 },
> +
> +	/* writ to unwrit splits */
> +	{ .desc = "split writ extent to 2 extents and convert 1st half unwrit (zeroout)",
> +	  .type = TEST_CREATE_BLOCKS,
> +	  .is_unwrit_at_start = 0,
> +	  .split_flags = EXT4_GET_BLOCKS_CONVERT_UNWRITTEN,
> +	  .split_map = { .m_lblk = 10, .m_len = 1 },
> +	  .nr_exp_ext = 1,
> +	  .exp_ext_state = { { .ex_lblk = 10, .ex_len = 3, .is_unwrit = 0 } },
> +	  .is_zeroout_test = 1,
> +	  .nr_exp_data_segs = 2,
> +	  .exp_data_state = { { .exp_char = 0, .off_blk = 0, .len_blk = 1 },
> +			      { .exp_char = 'X', .off_blk = 1, .len_blk = 2 }}},
> +	{ .desc = "split writ extent to 2 extents and convert 2nd half unwrit (zeroout)",
> +	  .type = TEST_CREATE_BLOCKS,
> +	  .is_unwrit_at_start = 0,
> +	  .split_flags = EXT4_GET_BLOCKS_CONVERT_UNWRITTEN,
> +	  .split_map = { .m_lblk = 11, .m_len = 2 },
> +	  .nr_exp_ext = 1,
> +	  .exp_ext_state = { { .ex_lblk = 10, .ex_len = 3, .is_unwrit = 0 } },
> +	  .is_zeroout_test = 1,
> +	  .nr_exp_data_segs = 2,
> +	  .exp_data_state = { { .exp_char = 'X', .off_blk = 0, .len_blk = 1 },
> +			      { .exp_char = 0, .off_blk = 1, .len_blk = 2 } } },
> +	{ .desc = "split writ extent to 3 extents and convert 2nd half unwrit (zeroout)",
> +	  .type = TEST_CREATE_BLOCKS,
> +	  .is_unwrit_at_start = 0,
> +	  .split_flags = EXT4_GET_BLOCKS_CONVERT_UNWRITTEN,
> +	  .split_map = { .m_lblk = 11, .m_len = 1 },
> +	  .nr_exp_ext = 1,
> +	  .exp_ext_state = { { .ex_lblk = 10, .ex_len = 3, .is_unwrit = 0 } },
> +	  .is_zeroout_test = 1,
> +	  .nr_exp_data_segs = 3,
> +	  .exp_data_state = { { .exp_char = 'X', .off_blk = 0, .len_blk = 1 },
> +			      { .exp_char = 0, .off_blk = 1, .len_blk = 1 },
> +			      { .exp_char = 'X', .off_blk = 2, .len_blk = 1 }}},
>   };
>   
>   static const struct kunit_ext_test_param test_handle_unwritten_params[] = {
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 8ade9c68ddd8..4c6e4e7a80b0 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -3463,6 +3463,15 @@ static struct ext4_ext_path *ext4_split_extent(handle_t *handle,
>   		 */
>   		goto out_orig_err;
>   
> +	if (flags & EXT4_GET_BLOCKS_CONVERT_UNWRITTEN) {
> +		int max_zeroout_blks =
> +			EXT4_SB(inode->i_sb)->s_extent_max_zeroout_kb >>
> +			(inode->i_sb->s_blocksize_bits - 10);
> +
> +		if (map->m_len > max_zeroout_blks)
> +			goto out_orig_err;
> +	}
> +
>   	path = ext4_find_extent(inode, map->m_lblk, NULL, flags);
>   	if (IS_ERR(path))
>   		goto out_orig_err;
> @@ -3818,15 +3827,10 @@ static struct ext4_ext_path *ext4_split_convert_extents(handle_t *handle,
>   		goto convert;
>   
>   	/*
> -	 * We don't use zeroout fallback for written to unwritten conversion as
> -	 * it is not as critical as endio and it might take unusually long.
> -	 * Also, it is only safe to convert extent to initialized via explicit
> +	 * It is only safe to convert extent to initialized via explicit
>   	 * zeroout only if extent is fully inside i_size or new_size.
>   	 */
> -	if (!(flags & EXT4_GET_BLOCKS_CONVERT_UNWRITTEN))
> -		split_flag |= ee_block + ee_len <= eof_block ?
> -				      EXT4_EXT_MAY_ZEROOUT :
> -				      0;
> +	split_flag |= ee_block + ee_len <= eof_block ? EXT4_EXT_MAY_ZEROOUT : 0;
>   
>   	/*
>   	 * pass SPLIT_NOMERGE explicitly so we don't end up merging extents we
> @@ -3948,7 +3952,20 @@ convert_initialized_extent(handle_t *handle, struct inode *inode,
>   
>   	ext4_update_inode_fsync_trans(handle, inode, 1);
>   
> -	map->m_flags |= EXT4_MAP_UNWRITTEN;
> +	/*
> +	 * The extent might be initialized in case of zeroout.
> +	 */
> +	path = ext4_find_extent(inode, map->m_lblk, path, flags);
> +	if (IS_ERR(path))
> +		return path;
> +
> +	depth = ext_depth(inode);
> +	ex = path[depth].p_ext;
> +
> +	if (ext4_ext_is_unwritten(ex))
> +		map->m_flags |= EXT4_MAP_UNWRITTEN;
> +	else
> +		map->m_flags |= EXT4_MAP_MAPPED;
>   	if (*allocated > map->m_len)
>   		*allocated = map->m_len;
>   	map->m_len = *allocated;