linux-ext4 - Re: lock i_mutex for fallocate?

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Message-ID: <20110901070848.GQ32358@dastard>
Date:	Thu, 1 Sep 2011 17:08:48 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Allison Henderson <achender@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	Andreas Dilger <adilger@...ger.ca>
Subject: Re: lock i_mutex for fallocate?

On Wed, Aug 31, 2011 at 05:33:25PM -0700, Allison Henderson wrote:
> Hi All,
> 
> In ext4 punch hole, we realized that the punch hole operation needs
> to be done under i_mutex just like truncate.  i_mutex for truncate
> is held in the vfs layer, so we dont need to lock it at the file
> system layer, but vfs does not lock i_mutex for fallocate.  We can
> lock i_mutex for fallocate at the fs layer, but question was raised
> then: should i_mutex for fallocate be held in the vfs layer instead?

No.

> I do not know if other file systems need i_mutex to be locked for
> fallocate,

For one, XFS does not require i_mutex to be held for any extent
manipulation of any kind (allocation, truncation, hole punch,
unwritten extent conversion, etc).

Hence the current structure of having the filesystem take i_mutex if
it needs it to protect allocations against races is appropriate.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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