linux-ext4 - Re: i_mutex questions

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Message-ID: <4E6FD4F0.4000901@linux.vnet.ibm.com>
Date:	Tue, 13 Sep 2011 15:10:56 -0700
From:	Allison Henderson <achender@...ux.vnet.ibm.com>
To:	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	"Ted Ts'o" <tytso@....edu>, jlbec@...lplan.org
Subject: Re: i_mutex questions

On 09/13/2011 12:02 PM, Joel Becker wrote:
> On Tue, Sep 13, 2011 at 11:33:29AM -0700, Allison Henderson wrote:
>> Hi All,
>>
>> I have been trying to find a way to synchronize punch hole with read
>> and write operations with out the use of i_mutex.  The concern is
>> that after punch hole has released the pages inside the hole,
>> another process may remap the page to a block before punch has taken
>> i_data_sem.  I think putting i_mutex around the punch hole operation
>> would fix this, but since we are trying to avoid further improper
>> use of i_mutex, I am trying to avoid that solution.
>
> Hey Allison,
> 	Actually, i_mutex is the normal way to handle this.  ocfs2 takes
> i_mutex down under its ->fallocate().  Truncate is in the same boat,
> which is why do_truncate() takes i_mutex before calling notify_change().
> 	The read-write paths grab i_mutex for buffered operation.  They
> don't for O_DIRECT, which doesn't map to the pagecache.  This is where
> i_data_sem should speed things up.
>
> Joel
>
Hi Joel,

Well, I actually already had a patch that was trying to use i_mutex to 
solve this ([PATCH 4/6 v7] ext4: Lock i_mutex for punch hole).  But we 
decided not to apply it because of plans to reduce the usage of i_mutex 
in the ext4 code.  So I've been trying to figure out a different way to 
solve this, but so far I haven't had a whole lot of luck finding a 
solution that doesn't involve introducing a new locking mechanism.  So I 
wanted to check back here for more details on what the plan for i_mutex 
is so I dont conflict with anything that might already be going on.  :)

Ted, would you be able to give us some more details on this topic?  Thx!

Allison Henderson
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