Re: [PATCH 6/6] xfs: refactor xfs_file_fallocate

[Dave Chinner <david@fromorbit.com>]

On Tue, Aug 27, 2024 at 08:50:50AM +0200, Christoph Hellwig wrote:
> Refactor xfs_file_fallocate into separate helpers for each mode,
> two factors for i_size handling and a single switch statement over the
> supported modes.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/xfs_file.c | 330 +++++++++++++++++++++++++++++-----------------
>  1 file changed, 208 insertions(+), 122 deletions(-)

Much nicer. :)

And it made an existing issue in the code quite obvious, too:

> +/*
> + * Punch a hole and prealloc the range.  We use a hole punch rather than
> + * unwritten extent conversion for two reasons:
> + *
> + *   1.) Hole punch handles partial block zeroing for us.
> + *   2.) If prealloc returns ENOSPC, the file range is still zero-valued by
> + *	 virtue of the hole punch.
> + */
> +static int
> +xfs_falloc_zero_range(
> +	struct file		*file,
> +	int			mode,
> +	loff_t			offset,
> +	loff_t			len)
> +{
> +	struct inode		*inode = file_inode(file);
> +	unsigned int		blksize = i_blocksize(inode);
> +	loff_t			new_size = 0;
> +	int			error;
> +
> +	trace_xfs_zero_file_space(XFS_I(inode));
> +
> +	error = xfs_falloc_newsize(file, mode, offset, len, &new_size);
> +	if (error)
> +		return error;
> +
> +	error = xfs_free_file_space(XFS_I(inode), offset, len);
> +	if (error)
> +		return error;
> +
> +	len = round_up(offset + len, blksize) - round_down(offset, blksize);
> +	offset = round_down(offset, blksize);
> +	error = xfs_alloc_file_space(XFS_I(inode), offset, len);
> +	if (error)
> +		return error;
> +	return xfs_falloc_setsize(file, new_size);
> +}

Our zeroing operation always does preallocation, but....


> +static int
> +xfs_falloc_allocate_range(
> +	struct file		*file,
> +	int			mode,
> +	loff_t			offset,
> +	loff_t			len)
> +{
> +	struct inode		*inode = file_inode(file);
> +	loff_t			new_size = 0;
> +	int			error;
> +
> +	/*
> +	 * If always_cow mode we can't use preallocations and thus should not
> +	 * create them.
> +	 */
> +	if (xfs_is_always_cow_inode(XFS_I(inode)))
> +		return -EOPNOTSUPP;

... our preallocation operation always returns -EOPNOTSUPP for
COW mode.

Should the zeroing code also have this COW mode check in it after
the hole punch has run so we don't do unnecessary prealloc there?

-Dave.
-- 
Dave Chinner
david@fromorbit.com