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

[Dave Chinner <david@fromorbit.com>]

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

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