linux-ext4 - Question about extents.c/ext4_ext_punch

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Date:	Sun, 15 Jul 2012 21:03:42 +0800
From:	Wang Sheng-Hui <shhuiw@...il.com>
To:	Allison Henderson <achender@...ux.vnet.ibm.com>,
	Lukas Czerner <lczerner@...hat.com>,
	"Theodore Ts'o" <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: Question about extents.c/ext4_ext_punch_hole

Dear all,

I'm reading the code of extents.c, and confused by the function "ext4_ext_punch_hole".

In my understanding, if we want to punch hole for the range (loff_t offset, loff_t length),
then we finally will get one hole block-size aligned, right?

[code in ext4_ext_punch_hole]
	first_block = (offset + sb->s_blocksize - 1) >>
		EXT4_BLOCK_SIZE_BITS(sb);
	stop_block = (offset + length) >> EXT4_BLOCK_SIZE_BITS(sb);
[code]

>From the code, we'll get one hole range contained in the range [offset, length+offset-1].

For example, we set the block size to 1K, and we specify the range [offset:1000, offset:2000].
Then we'll get first_block:1,stop_block:2, with some head/tail range ignored.
      {-23------|===========|----952---}
     1000      1024       2048        2999
     offset                        offset+length-1
'==='means punched hole here.

Please correct me if I'm wrong.


Regards,
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