linux-ext4 - Re: One question about ext4

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Message-Id: <A9962DF5-1C88-4385-A8DA-20227E055910@dilger.ca>
Date:	Thu, 19 Jul 2012 08:42:12 -0700
From:	Andreas Dilger <aedilger@...il.com>
To:	Wang Sheng-Hui <shhuiw@...il.com>
Cc:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: One question about ext4_fallocate


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





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