One question about ext4_fallocate

One question about ext4_fallocate

[Wang Sheng-Hui]

Dear all,

I have one question about ext4_fallocate:
	For its parameters, offset & len, it will preallocate
	one space with length >= len, right?

>From the code:
	map.m_lblk = offset >> blkbits;
	/*
	 * We can't just convert len to max_blocks because
	 * If blocksize = 4096 offset = 3072 and len = 2048
	 */
	max_blocks = (EXT4_BLOCK_ALIGN(len + offset, blkbits) >> blkbits)
		- map.m_lblk;

We can see that the target space is enlarged to the (floor, ceiling) of 
(offset, offset+len). Right?

If I'm wrong, please correct me.


Regards,

Re: One question about ext4_fallocate

[Andreas Dilger]

On 2012-07-19, at 7:51 AM, Wang Sheng-Hui wrote:
> I have one question about ext4_fallocate:
> 	For its parameters, offset & len, it will preallocate
> 	one space with length >= len, right?
> 
> From the code:
> 	map.m_lblk = offset >> blkbits;
> 	/*
> 	 * We can't just convert len to max_blocks because
> 	 * If blocksize = 4096 offset = 3072 and len = 2048
> 	 */
> 	max_blocks = (EXT4_BLOCK_ALIGN(len + offset, blkbits) >> blkbits)
> 		- map.m_lblk;
> 
> We can see that the target space is enlarged to the (floor, ceiling) of 
> (offset, offset+len). Right?
> 
> If I'm wrong, please correct me.

A whole block has to be allocated for fallocate, but if KEEP_SIZE is not used, the i_size should be changed to match the actual requested size.

Cheers, Andreas

Re: One question about ext4_fallocate

[Wang Sheng-Hui]

On 2012年07月19日 23:42, Andreas Dilger wrote:
> 
> On 2012-07-19, at 7:51 AM, Wang Sheng-Hui wrote:
>> I have one question about ext4_fallocate:
>> 	For its parameters, offset & len, it will preallocate
>> 	one space with length >= len, right?
>>
>> From the code:
>> 	map.m_lblk = offset >> blkbits;
>> 	/*
>> 	 * We can't just convert len to max_blocks because
>> 	 * If blocksize = 4096 offset = 3072 and len = 2048
>> 	 */
>> 	max_blocks = (EXT4_BLOCK_ALIGN(len + offset, blkbits) >> blkbits)
>> 		- map.m_lblk;
>>
>> We can see that the target space is enlarged to the (floor, ceiling) of 
>> (offset, offset+len). Right?
>>
>> If I'm wrong, please correct me.
> 
> A whole block has to be allocated for fallocate, but if KEEP_SIZE is not used, the i_size should be changed to match the actual requested size.
> 

Got it.

From the code:
	/* Return error if mode is not supported */
	if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE))
		return -EOPNOTSUPP;

	if (mode & FALLOC_FL_PUNCH_HOLE)
		return ext4_punch_hole(file, offset, len);

ext4 only supports FALLOC_FL_KEEP_SIZE & FALLOC_FL_PUNCH_HOLE currently.
For FALLOC_FL_PUNCH_HOLE, it will turn to ext4_punch_hole, so the rest
path deals with only FALLOC_FL_KEEP_SIZE and thus it will use whole blocks.

Thanks, Andreas.
> Cheers, Andreas
> 
> 
> 
> 
> 

--
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html