Re: [PATCH 1/6] fs: add hole punching to fallocate

[Dave Chinner <david@fromorbit.com>]

On Mon, Nov 08, 2010 at 03:32:02PM -0500, Josef Bacik wrote:
> Hole punching has already been implemented by XFS and OCFS2, and has the
> potential to be implemented on both BTRFS and EXT4 so we need a generic way to
> get to this feature.  The simplest way in my mind is to add FALLOC_FL_PUNCH_HOLE
> to fallocate() since it already looks like the normal fallocate() operation.
> I've tested this patch with XFS and BTRFS to make sure XFS did what it's
> supposed to do and that BTRFS failed like it was supposed to.  Thank you,
> 
> Signed-off-by: Josef Bacik <josef@redhat.com>
> ---
>  fs/open.c              |    2 +-
>  include/linux/falloc.h |    1 +
>  2 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/open.c b/fs/open.c
> index 4197b9e..ab8dedf 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -223,7 +223,7 @@ int do_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
>  		return -EINVAL;
>  
>  	/* Return error if mode is not supported */
> -	if (mode && !(mode & FALLOC_FL_KEEP_SIZE))
> +	if (mode && (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE)))
>  		return -EOPNOTSUPP;
>  
>  	if (!(file->f_mode & FMODE_WRITE))
> diff --git a/include/linux/falloc.h b/include/linux/falloc.h
> index 3c15510..851cba2 100644
> --- a/include/linux/falloc.h
> +++ b/include/linux/falloc.h
> @@ -2,6 +2,7 @@
>  #define _FALLOC_H_
>  
>  #define FALLOC_FL_KEEP_SIZE	0x01 /* default is extend size */
> +#define FALLOC_FL_PUNCH_HOLE	0X02 /* de-allocates range */

Hole punching was not included originally in fallocate() for a
variety of reasons. IIRC, they were along the lines of:

	1 de-allocating of blocks in an allocation syscall is wrong.
	  People wanted a new syscall for this functionality.
	2 no glibc interface needs it
	3 at the time, only XFS supported punching holes, so there
	  is not need to support it in a generic interface
	4 the use cases presented were not considered compelling
	  enough to justify the additional complexity (!)

In the end, I gave up arguing for it to be included because just
getting the FALLOC_FL_KEEP_SIZE functionality was a hard enough
battle.

Anyway, #3 isn't the case any more, #4 was just an excuse not to
support anything ext4 couldn't do and lots of apps are calling
fallocate directly (because glibc can't use FALLOC_FL_KEEP_SIZE) so
#2 isn't an issue, either. I guess that leaves #1 to be debated;
I don't think there is any problem with doing what you propose.

What I will suggest is that this requires a generic xfstest to be
written and support added to xfs_io to enable that test (and others)
to issue hole punches. Something along the lines of test 242 which I
wrote for testing all the edge case of XFS_IOC_ZERO_RANGE (*) would be
good.

Cheers,

Dave.

(*) fallocate() version:
http://git.kernel.org/?p=linux/kernel/git/dgc/xfsdev.git;a=commitdiff;h=45f3e1831e3abc8bd12ec1e6c548f73a8dd9e36d
-- 
Dave Chinner
david@fromorbit.com