linux-kernel - Re: [PATCH -next v2 1/7] ext4: use reserved metadata blocks when splitting extent on endio

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Message-ID: <aVkc27cw-m5Skeo0@li-dc0c254c-257c-11b2-a85c-98b6c1322444.ibm.com>
Date: Sat, 3 Jan 2026 19:12:51 +0530
From: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To: Zhang Yi <yi.zhang@...weicloud.com>
Cc: linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca,
        jack@...e.cz, ritesh.list@...il.com, yi.zhang@...wei.com,
        yizhang089@...il.com, libaokun1@...wei.com, yangerkun@...wei.com,
        yukuai@...as.com
Subject: Re: [PATCH -next v2 1/7] ext4: use reserved metadata blocks when
 splitting extent on endio

On Tue, Dec 23, 2025 at 09:17:56AM +0800, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@...wei.com>
> 
> When performing buffered writes, we may need to split and convert an
> unwritten extent into a written one during the end I/O process. However,
> we do not reserve space specifically for these metadata changes, we only
> reserve 2% of space or 4096 blocks. To address this, we use
> EXT4_GET_BLOCKS_PRE_IO to potentially split extents in advance and
> EXT4_GET_BLOCKS_METADATA_NOFAIL to utilize reserved space if necessary.
> 
> These two approaches can reduce the likelihood of running out of space
> and losing data. However, these methods are merely best efforts, we
> could still run out of space, and there is not much difference between
> converting an extent during the writeback process and the end I/O
> process, it won't increase the rick of losing data if we postpone the
                                ^^^^ risk

Other than the minor typo above, feel free to add:
Reviewed-by: Ojaswin Mujoo <ojaswin@...ux.ibm.com>

> conversion.
> 
> Therefore, also use EXT4_GET_BLOCKS_METADATA_NOFAIL in
> ext4_convert_unwritten_extents_endio() to prepare for the buffered I/O
> iomap conversion, which may perform extent conversion during the end I/O
> process.
> 
> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
> Reviewed-by: Jan Kara <jack@...e.cz>

> ---
>  fs/ext4/extents.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 27eb2c1df012..e53959120b04 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -3794,6 +3794,8 @@ ext4_convert_unwritten_extents_endio(handle_t *handle, struct inode *inode,
>  	 * illegal.
>  	 */
>  	if (ee_block != map->m_lblk || ee_len > map->m_len) {
> +		int flags = EXT4_GET_BLOCKS_CONVERT |
> +			    EXT4_GET_BLOCKS_METADATA_NOFAIL;
>  #ifdef CONFIG_EXT4_DEBUG
>  		ext4_warning(inode->i_sb, "Inode (%ld) finished: extent logical block %llu,"
>  			     " len %u; IO logical block %llu, len %u",
> @@ -3801,7 +3803,7 @@ ext4_convert_unwritten_extents_endio(handle_t *handle, struct inode *inode,
>  			     (unsigned long long)map->m_lblk, map->m_len);
>  #endif
>  		path = ext4_split_convert_extents(handle, inode, map, path,
> -						EXT4_GET_BLOCKS_CONVERT, NULL);
> +						  flags, NULL);
>  		if (IS_ERR(path))
>  			return path;
>  
> -- 
> 2.52.0
>