Re: [PATCH v4 17/23] ext4: submit zeroed post-EOF data immediately in the iomap buffered I/O path

[Zhang Yi <yizhang089@gmail.com>]

On 6/18/2026 8:59 PM, Jan Kara wrote:
> On Mon 11-05-26 15:23:37, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@huawei.com>
>>
>> In the generic buffered_head I/O path, we rely on the data=order mode to
>> ensure that the zeroed EOF block data is written before updating
>> i_disksize, thus preventing stale data from being exposed.
>>
>> However, the iomap buffered I/O path cannot use this mechanism. Instead,
>> we issue the I/O immediately after performing the zero operation
>> (without synchronous waiting for performance). This can reduce the risk
>> of exposing stale data, but it does not guarantee that the zero data
>> will be flushed to disk before the metadata of i_disksize is updated.
>> The subsequent patches will wait for this I/O to complete before
>> updating i_disksize.
>>
>> Suggested-by: Jan Kara <jack@suse.cz>
>> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
> 
> Two nits below:
> 
>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>> index 239d387ffaf2..e013aeb03d7b 100644
>> --- a/fs/ext4/inode.c
>> +++ b/fs/ext4/inode.c
>> @@ -4742,6 +4742,32 @@ static int ext4_block_zero_range(struct inode *inode,
>>   					zero_written);
>>   }
>>   
>> +static int ext4_iomap_submit_zero_block(struct inode *inode,
>> +					loff_t from, loff_t end)
>> +{
>> +	struct address_space *mapping = inode->i_mapping;
>> +	struct folio *folio;
>> +	bool do_submit = false;
>> +
>> +	folio = filemap_lock_folio(mapping, from >> PAGE_SHIFT);
>> +	if (IS_ERR(folio))
>> +		/* Already writeback and clear? */
> 		   ^^^ Already written back and reclaimed
> 
>> +		return PTR_ERR(folio) == -ENOENT ? 0 : PTR_ERR(folio);
>> +
>> +	folio_wait_writeback(folio);
>> +	WARN_ON_ONCE(folio_test_writeback(folio));
>> +
>> +	if (likely(folio_test_dirty(folio)))
>> +		do_submit = true;
>> +	folio_unlock(folio);
>> +	folio_put(folio);
> 
> So how is what you do here more efficient than just:
> 
> 	filemap_fdatawrite_range(mapping, from, end - 1)
> 
> ? That will also do nothing if the folio isn't dirty, won't it?
> 
> 								Honza

Yeah, we can just call filemap_fdatawrite_range() in this patch. These
logic for locking/unlocking and checking writeback is only needed in
patch 18, and should be moved to a later patch. It is used to avoid
setting ordered block on folios that have already been concurrently
written back. Sorry for confusing.

Thanks,
Yi.

> 
>> +
>> +	/* Submit zeroed block. */
>> +	if (do_submit)
>> +		return filemap_fdatawrite_range(mapping, from, end - 1);
>> +	return 0;
>> +}
>> +
>>   /*
>>    * Zero out a mapping from file offset 'from' up to the end of the block
>>    * which corresponds to 'from' or to the given 'end' inside this block.
>> @@ -4765,8 +4791,10 @@ int ext4_block_zero_eof(struct inode *inode, loff_t from, loff_t end)
>>   	if (IS_ENCRYPTED(inode) && !fscrypt_has_encryption_key(inode))
>>   		return 0;
>>   
>> -	if (length > blocksize - offset)
>> +	if (length > blocksize - offset) {
>>   		length = blocksize - offset;
>> +		end = from + length;
>> +	}
>>   
>>   	err = ext4_block_zero_range(inode, from, length,
>>   				    &did_zero, &zero_written);
>> @@ -4781,18 +4809,34 @@ int ext4_block_zero_eof(struct inode *inode, loff_t from, loff_t end)
>>   	 * TODO: In the iomap path, handle this by updating i_disksize to
>>   	 * i_size after the zeroed data has been written back.
>>   	 */
>> -	if (ext4_should_order_data(inode) &&
>> -	    did_zero && zero_written && !IS_DAX(inode)) {
>> -		handle_t *handle;
>> +	if (did_zero && zero_written && !IS_DAX(inode)) {
>> +		if (ext4_should_order_data(inode)) {
>> +			handle_t *handle;
>>   
>> -		handle = ext4_journal_start(inode, EXT4_HT_MISC, 1);
>> -		if (IS_ERR(handle))
>> -			return PTR_ERR(handle);
>> +			handle = ext4_journal_start(inode, EXT4_HT_MISC, 1);
>> +			if (IS_ERR(handle))
>> +				return PTR_ERR(handle);
>>   
>> -		err = ext4_jbd2_inode_add_write(handle, inode, from, length);
>> -		ext4_journal_stop(handle);
>> -		if (err)
>> -			return err;
>> +			err = ext4_jbd2_inode_add_write(handle, inode, from,
>> +							length);
>> +			ext4_journal_stop(handle);
>> +			if (err)
>> +				return err;
>> +		/*
>> +		 * inodes using the iomap buffered I/O path do not use the
>> +		 * data=ordered mode. We submit zeroed range directly here.
>> +		 * Do not wait for I/O completion for performance.
>> +		 *
>> +		 * TODO: Any operation that extends i_disksize (including
>> +		 * append write end io past the zeroed boundary, truncate up,
>> +		 * and append fallocate) must wait for the relevant I/O to
>> +		 * complete before updating i_disksize.
>> +		 */
>> +		} else if (ext4_inode_buffered_iomap(inode)) {
>> +			err = ext4_iomap_submit_zero_block(inode, from, end);
>> +			if (err)
>> +				return err;
>> +		}
>>   	}
>>   
>>   	return 0;
>> -- 
>> 2.52.0
>>