Re: [PATCH v3 3/7] xfs: cap prealloc size to free space before shift

Dave Chinner
2013-02-19T21:48:47Z

On Tue, Feb 19, 2013 at 11:37:27AM -0500, Brian Foster wrote:
> With the addition of quota preallocation throttling, we want to
> support a general algorithm that considers the maximum allowable
> prealloc size and recommended shift modifier from various sources
> (i.e., global fs state and all applicable quotas for an inode).
>
> Update the current global free space throttle algorithm to cap the
> preallocation size to the free space available in the filesystem.
>
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
> fs/xfs/xfs_iomap.c | 3 +++
> 1 files changed, 3 insertions(+), 0 deletions(-)
>
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index daa08f6..3b41c18 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -412,6 +412,9 @@ xfs_iomap_prealloc_size(
> if (freesp < mp->m_low_space[XFS_LOWSP_1_PCNT])
> shift++;
> }
> + if (alloc_blocks > freesp)
> + alloc_blocks = freesp;
> +
> if (shift)
> alloc_blocks >>= shift;

This is redundant with the previous additions of the trailing

while (alloc_blocks >= freesp)
alloc_blocks >>= 4;

code. Effectively adding the check will result in preventing the
existing loop from working as alloc_blocks will be brought down to
just under freespc by things like power of 2 rounding, rather than
being thottled to a small fraction of the remaining free space...

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com