Re: [PATCH] Btrfs: btrfs_defrag_file: Fix calculation of max_to_defrag.

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On Wed, Jun 03, 2015 at 02:59:54PM +0530, Chandan Rajendra wrote:
> max_to_defrag represents the number of pages to defrag rather than the last
> page of the file range to be defragged. Fix this.

Please update the changelog and describe the buggy behaviour. From a
brief look I think that the defrag batch will be much higher than
desired. Elsewhere the defrag batch is 1024, the page indexes used here
are always large numbers. I guess this could (partially) explain high
load during defrag that people report.

ACK for the fix, I'll stick a reviewed-by to v2.

> 
> Signed-off-by: Chandan Rajendra <chandan@xxxxxxxxxxxxxxxxxx>
> ---
>  fs/btrfs/ioctl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index ca5d968..2a45026 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -1318,7 +1318,7 @@ int btrfs_defrag_file(struct inode *inode, struct file *file,
>  		i = range->start >> PAGE_CACHE_SHIFT;
>  	}
>  	if (!max_to_defrag)
> -		max_to_defrag = last_index + 1;
> +		max_to_defrag = last_index - i + 1;
>  
>  	/*
>  	 * make writeback starts from i, so the defrag range can be
> -- 
> 2.1.0
> 
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