Re: [PATCH] Btrfs: fix defrag to merge tail file extent

[Liu Bo <bo.li.liu@oracle.com>]

Writing a xfstests case for this, will come soon.

Thanks,

-liubo

On Fri, Aug 07, 2015 at 04:48:41PM +0800, Liu Bo wrote:
> The file layout is
> 
> [extent 1]...[extent n][4k extent][HOLE][extent x]
> 
> extent 1~n and 4k extent can be merged during defrag, and the whole
> defrag bytes is larger than our defrag thresh(256k), 4k extent as a
> tail is left unmerged since we check if its next extent can be merged
> (the next one is a hole, so the check will fail), the layout thus can
> be
> 
> [new extent][4k extent][HOLE][extent x]
>  (1~n)
> 
> To fix it, beside looking at the next one, this also looks at the
> previous one by checking @defrag_end, which is set to 0 when we
> decide to stop merging contiguous extents, otherwise, we can merge
> the previous one with our extent.
> 
> Also, this makes btrfs behave consistent with how xfs and ext4 do.
> 
> Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
> ---
>  fs/btrfs/ioctl.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index 0770c91..0691b2b 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -1030,6 +1030,7 @@ static int should_defrag_range(struct inode *inode, u64 start, u32 thresh,
>  	struct extent_map *em;
>  	int ret = 1;
>  	bool next_mergeable = true;
> +	bool prev_mergeable = true;
>  
>  	/*
>  	 * make sure that once we start defragging an extent, we keep on
> @@ -1050,13 +1051,16 @@ static int should_defrag_range(struct inode *inode, u64 start, u32 thresh,
>  		goto out;
>  	}
>  
> +	if (!*defrag_end)
> +		prev_mergeable = false;
> +
>  	next_mergeable = defrag_check_next_extent(inode, em);
>  	/*
>  	 * we hit a real extent, if it is big or the next extent is not a
>  	 * real extent, don't bother defragging it
>  	 */
>  	if (!compress && (*last_len == 0 || *last_len >= thresh) &&
> -	    (em->len >= thresh || !next_mergeable))
> +	    (em->len >= thresh || (!next_mergeable && !prev_mergeable)))
>  		ret = 0;
>  out:
>  	/*
> -- 
> 2.1.0
> 
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