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Date:	Tue, 16 Nov 2010 22:06:40 -0500
From:	Ted Ts'o <tytso@....edu>
To:	Avi Kivity <avi@...hat.com>
Cc:	Josef Bacik <josef@...hat.com>, david@...morbit.com,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	xfs@....sgi.com, cmm@...ibm.com, cluster-devel@...hat.com,
	ocfs2-devel@....oracle.com
Subject: Re: [PATCH 4/6] Ext4: fail if we try to use hole punch

> >There is no simple way to test if a filesystem supports hole punching or not so
> >the check has to be done per fs.  Thanks,
> 
> Could put a flag word in superblock_operations.  Filesystems which
> support punching (or other features) can enable it there.

No, it couldn't be in super_operations.  It may vary on a per-inode
basis for some file systems, such as ext4 (depending on whether the
inode is extent-mapped or indirect-block mapped).

So at least for ext4 we'd need to call into fallocate() function
anyway, once we add support.  I suppose if other file systems really
want it, we could add a flag to the super block ops structure, so they
don't have do the "do we support the punch" operation.  I can go
either way on that; although if we think the majority of file systems
are going support punch in the long-term, then it might not be worth
it to add such a flag.

						- Ted
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