[PATCH v2 2/7] xfs: push rounddown_pow_of_two() to after prealloc throttle

[Ben Myers]

Hey Brian,

On Wed, Jan 02, 2013 at 01:08:06PM -0500, Brian Foster wrote:
> The round down occurs towards the beginning of the function. Push
> it down after throttling has occurred. This is to support adding > further transformations to 'alloc_blocks' that might not preserve
> power-of-two alignment (and thus could lead to rounding down
> multiple times).
> 
> Signed-off-by: Brian Foster <bfoster at redhat.com>
> ---
>  fs/xfs/xfs_iomap.c |   21 +++++++++++++++------
>  1 files changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index bd7c060..b2002a5 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -329,13 +329,14 @@ xfs_iomap_prealloc_size(
>  		goto check_writeio;
>  
>  	/*
> -	 * rounddown_pow_of_two() returns an undefined result
> -	 * if we pass in alloc_blocks = 0. Hence the "+ 1" to
> -	 * ensure we always pass in a non-zero value.
> +	 * MAXEXTLEN is not a power of two value but we round the prealloc down
> +	 * to the nearest power of two value after throttling. To prevent the
> +	 * round down from unconditionally reducing the maximum supported prealloc
> +	 * size, we round up first, apply appropriate throttling, round down and
> +	 * cap the value to MAXEXTLEN.
>  	 */
> -	alloc_blocks = XFS_B_TO_FSB(mp, XFS_ISIZE(ip)) + 1;
> -	alloc_blocks = XFS_FILEOFF_MIN(MAXEXTLEN,
> -				rounddown_pow_of_two(alloc_blocks));
> +	alloc_blocks = XFS_FILEOFF_MIN(roundup_pow_of_two(MAXEXTLEN),
> +				XFS_B_TO_FSB(mp, XFS_ISIZE(ip)));
>  
>  	xfs_icsb_sync_counters(mp, XFS_ICSB_LAZY_COUNT);
>  	freesp = mp->m_sb.sb_fdblocks;
> @@ -352,6 +353,14 @@ xfs_iomap_prealloc_size(
>  	}
>  	if (shift)
>  		alloc_blocks >>= shift;
> +	/*
> +	 * rounddown_pow_of_two() returns an undefined result if we pass in
> +	 * alloc_blocks = 0.
> +	 */
> +	if (alloc_blocks)
> +		alloc_blocks = rounddown_pow_of_two(alloc_blocks);
> +	if (alloc_blocks > MAXEXTLEN)
> +		alloc_blocks = MAXEXTLEN;

Well that hurt my brain cell.  It turns out:

xfs_fsblock_t   len, len2, len3;
len = MAXEXTLEN;
len2 = roundup_pow_of_two(len);
len3 = rounddown_pow_of_two(len2);

printk("maxextlen %lu up one %lu down one %lu\n", len, len2, len3);

prints:
maxextlen 2097151 up one 2097152 down one 2097152

Looks good Brian.

Reviewed-by: Ben Myers <bpm at sgi.com>