Re: [PATCH v6 08/31] ext4: avoid unnecessary transaction in ext4_map_blocks() for unwritten extents

[Ojaswin Mujoo]

On Thu, Sep 03, 2026 at 08:35:20PM +0800, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
> 
> When ext4_map_blocks() finds an unwritten extent in the extent cache and
> the caller is willing to accept unwritten extents without conversion,
> there is no need to start a journal transaction since no metadata update
> is required. This avoids unnecessary transaction overhead in the
> upcoming iomap writeback path when overwriting already-allocated
> unwritten extents.
> 
> One thing to be careful about, as the comment in ext4_map_blocks()
> states, if the flags contain EXT4_GET_BLOCKS_CREATE, the function will
> mark @map as mapped.
> 
> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>

Yeahh, I had something similar in one of my WIP branches :)

Thanks for taking care of this. Feel free to add:

Reviewed-by: Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx>

Regards,
ojaswin

> ---
>  fs/ext4/inode.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index c9dea4ca5caf..7a5c74af8ff3 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -809,14 +809,23 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
>  	 * Note that if blocks have been preallocated
>  	 * ext4_ext_map_blocks() returns with buffer head unmapped
>  	 */
> -	if (retval > 0 && map->m_flags & EXT4_MAP_MAPPED)
> +	if (retval > 0) {
>  		/*
> -		 * If we need to convert extent to unwritten
> -		 * we continue and do the actual work in
> -		 * ext4_ext_map_blocks()
> +		 * If we need to convert written extent to unwritten or
> +		 * convert unwritten extent to written, continue and do
> +		 * the actual work in ext4_ext_map_blocks().
>  		 */
> -		if (!(flags & EXT4_GET_BLOCKS_CONVERT_UNWRITTEN))
> +		if (map->m_flags & EXT4_MAP_MAPPED &&
> +		    !(flags & EXT4_GET_BLOCKS_CONVERT_UNWRITTEN))
>  			goto out_handle;
> +		if (map->m_flags & EXT4_MAP_UNWRITTEN &&
> +		    (flags & EXT4_GET_BLOCKS_UNWRIT_EXT) &&
> +		    !(flags & EXT4_GET_BLOCKS_CONVERT)) {
> +			/* Contains EXT4_GET_BLOCKS_CREATE - mark mapped. */
> +			map->m_flags |= EXT4_MAP_MAPPED;
> +			goto out_handle;
> +		}
> +	}
>  
>  	if (!handle) {
>  		handle = ext4_journal_start(inode, EXT4_HT_MAP_BLOCKS,
> -- 
> 2.52.0
>