linux-ext4 - Re: One question about ext4

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Date:	Fri, 20 Jul 2012 14:39:23 +0800
From:	Wang Sheng-Hui <shhuiw@...il.com>
To:	Andreas Dilger <aedilger@...il.com>
CC:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: One question about ext4_fallocate

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
> 
> 
> 
> 
> 

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