linux-kernel - Re: [PATCH 2/6] XFS: handle hole punching via fallocate properly

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Message-ID: <20101109012254.GE2715@dastard>
Date:	Tue, 9 Nov 2010 12:22:54 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Josef Bacik <josef@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	xfs@....sgi.com, joel.becker@...cle.com, cmm@...ibm.com,
	cluster-devel@...hat.com
Subject: Re: [PATCH 2/6] XFS: handle hole punching via fallocate properly

On Mon, Nov 08, 2010 at 03:32:03PM -0500, Josef Bacik wrote:
> This patch simply allows XFS to handle the hole punching flag in fallocate
> properly.  I've tested this with a little program that does a bunch of random
> hole punching with FL_KEEP_SIZE and without it to make sure it does the right
> thing.  Thanks,
> 
> Signed-off-by: Josef Bacik <josef@...hat.com>
> ---
>  fs/xfs/linux-2.6/xfs_iops.c |   12 +++++++++---
>  1 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/xfs/linux-2.6/xfs_iops.c b/fs/xfs/linux-2.6/xfs_iops.c
> index 96107ef..99df347 100644
> --- a/fs/xfs/linux-2.6/xfs_iops.c
> +++ b/fs/xfs/linux-2.6/xfs_iops.c
> @@ -516,6 +516,7 @@ xfs_vn_fallocate(
>  	loff_t		new_size = 0;
>  	xfs_flock64_t	bf;
>  	xfs_inode_t	*ip = XFS_I(inode);
> +	int		cmd = XFS_IOC_RESVSP;
>  
>  	/* preallocation on directories not yet supported */
>  	error = -ENODEV;
> @@ -528,17 +529,22 @@ xfs_vn_fallocate(
>  
>  	xfs_ilock(ip, XFS_IOLOCK_EXCL);
>  
> +	if (mode & FALLOC_FL_PUNCH_HOLE)
> +		cmd = XFS_IOC_UNRESVSP;
> +
>  	/* check the new inode size is valid before allocating */
>  	if (!(mode & FALLOC_FL_KEEP_SIZE) &&
>  	    offset + len > i_size_read(inode)) {
> -		new_size = offset + len;
> +		if (cmd == XFS_IOC_UNRESVSP)
> +			new_size = offset;
> +		else
> +			new_size = offset + len;

What semantic is FALLOC_FL_KEEP_SIZE supposed to convey during a
hole punch? Doesn't this just turn the hole punch operation into
a truncate? If so, what's the point of punching the hole when you
can just call ftruncate() to get the same result?

I'd suggest that FALLOC_FL_PUNCH_HOLE should, by definition, not
change the file size, and have no option to be able to change it.
This needs to be defined and documented - can you include a man
page update in this series that defines the expected behaviour
of FALLOC_FL_PUNCH_HOLE?

Cheers,

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