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

[Zhang Yi <yizhang089@gmail.com>]

On 9/28/2026 6:21 PM, Ojaswin Mujoo wrote:
> On Thu, Sep 03, 2026 at 08:35:20PM +0800, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@huawei.com>
>>
>> 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@huawei.com>
> 
> 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@linux.ibm.com>
> 
> Regards,
> ojaswin

Hi ojaswin,

Thank you for the review! The fast path implemented in the current
version is missing check_block_validity(), I'll add it in the next
iteration.

Thanks,
Yi.

> 
>> ---
>>   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
>>