Re: [PATCH] ext4: don't order data when zeroing unwritten or delayed block

[Zhang Yi]

On 12/22/2025 6:48 PM, Jan Kara wrote:
> On Mon 22-12-25 09:31:36, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
>>
>> When zeroing out a written partial block, it is necessary to order the
>> data to prevent exposing stale data on disk. However, if the buffer is
>> unwritten or delayed, it is not allocated as written, so ordering the
>> data is not required. This can prevent strange and unnecessary ordered
>> writes when appending data across a region within a block.
>>
>> Assume we have a 2K unwritten file on a filesystem with 4K blocksize,
>> and buffered write from 3K to 4K. Before this patch,
>> __ext4_block_zero_page_range() would add the range [2k,3k) to the
>> ordered range, and then the JBD2 commit process would write back this
>> block. However, it does nothing since the block is not mapped, this
> 							^^^ by this you
> mean that the block is unwritten, don't you?
> 

Yes, that is exactly what I wanted to express. The term "not mapped" might
indeed be unclear and prone to misunderstanding. I will revise it to "the
block is not mapped as written" in v2.

Thanks,
Yi.

>> folio will be redirtied and written back agian through the normal write
>> back process.
>>
>> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
> 
> The patch looks good. Feel free to add:
> 
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> 
> 								Honza
> 
>> ---
>>  fs/ext4/inode.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>> index fa579e857baf..fc16a89903b9 100644
>> --- a/fs/ext4/inode.c
>> +++ b/fs/ext4/inode.c
>> @@ -4104,9 +4104,13 @@ static int __ext4_block_zero_page_range(handle_t *handle,
>>  	if (ext4_should_journal_data(inode)) {
>>  		err = ext4_dirty_journalled_data(handle, bh);
>>  	} else {
>> -		err = 0;
>>  		mark_buffer_dirty(bh);
>> -		if (ext4_should_order_data(inode))
>> +		/*
>> +		 * Only the written block requires ordered data to prevent
>> +		 * exposing stale data.
>> +		 */
>> +		if (!buffer_unwritten(bh) && !buffer_delay(bh) &&
>> +		    ext4_should_order_data(inode))
>>  			err = ext4_jbd2_inode_add_write(handle, inode, from,
>>  					length);
>>  	}
>> -- 
>> 2.52.0
>>