linux-ext4 - Re: [RFC][PATCH 09/10 v1][RESEND] ext4: don't need to writeout all dirty pages in punch hole

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Date:	Mon, 23 Jul 2012 21:18:01 +0800
From:	Zheng Liu <gnehzuil.liu@...il.com>
To:	Lukáš Czerner <lczerner@...hat.com>
Cc:	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	xiaoqiangnk@...il.com, achender@...ux.vnet.ibm.com,
	wenqing.lz@...bao.com
Subject: Re: [RFC][PATCH 09/10 v1][RESEND] ext4: don't need to writeout all
 dirty pages in punch hole

On Mon, Jul 23, 2012 at 02:20:57PM +0200, Lukáš Czerner wrote:
[cut...]
> > > Moreover I think that we should avoid taking i_mutex if we can and I
> > > believe that we can in this case, because we only need to prevent
> > > allocation. So I just want to let you know that this part is
> > > probably going to change anyway.
> > 
> > It seems that we need to take i_mutex locking to prevent from buffered
> > writes after page cache has been truncated by truncate_pagecache_range.
> > If a buffered write without delalloc occurs after truncating page cache
> > and before taking i_data_sem, that means that the allocated block for
> > this buffered write will be removed in ext4_ext_remove_space when the
> > offset is within the range of the hole.  Am I missing something?
> 
> You're absolutely right, currently this is possible. But I think that we
> can take i_data_sem before truncating the pagecache hence preventing anyone
> from mapping new blocks. However this is not yet implemented in my
> patch set.
> 
> ...
> hmm, looking at the ext4_write_begin() it seems like it might not be
> such good idea after all. It seems to take page lock before
> i_data_sem so we might get deadlock, moreover if the punch hole
> happened in the middle of the ext4_write_begin() we might have only
> part of the data written, moreover this does not have to be hole
> aligned, which is bad. I need to revise that.

Yes, this is why I think that i_mutex locking should be taken.  At least
we are safty when we take the i_mutex. :-)

Regards,
Zheng
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