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

[Zhang Yi]

On 1/3/2026 9:42 PM, Ojaswin Mujoo wrote:
> On Tue, Dec 23, 2025 at 09:17:56AM +0800, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
>>
>> 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@xxxxxxxxxxxxx>

Thank you for reviewing this series, I will revise it in v3.

Cheers,
Yi.

> 
>> 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@xxxxxxxxxx>
>> Reviewed-by: Jan Kara <jack@xxxxxxx>
> 
>> ---
>>  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
>>