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@xxxxxxxxxx>
> ---
> 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@xxxxxxx>
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