linux-ext4 - Re: Question on delalloc

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Message-ID: <20130604052203.GA29587@thunk.org>
Date:	Tue, 4 Jun 2013 01:22:03 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	宋柏翰 <solarispika@...il.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: Question on delalloc

On Tue, Jun 04, 2013 at 03:00:29AM +0800, 宋柏翰 wrote:
> Also after I traced into filemap_write_and_wait_range, I found it
> eventually calls ext4_writepage to do the most part of work, which in
> its comment says "We don't do any block allocation in this function."
> 
> So, can anyone kindly shed any light on my question, or point out my mistakes?

There are three possible address_space_operations structures that can
be used for ext4 files.  They are ext4_aops, ext4_journalled_aops, and
ext4_da_aops.  It is the last one which is used for delayed allocation
files, and in that case filemap_write_and_wait_range will use
ext4_da_writepages().

These days, if there is an writepages function, the writepage function
is not used at all.  It used to be used for direct reclaim, but that's
been replaced by I/O-less reclaim.

						- Ted
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