Re: [PATCH] xfs: Make fiemap works with sparse file.

[Eric Sandeen]

Tao Ma wrote:
> In xfs_vn_fiemap, we set bvm_count to fi_extent_max + 1 and want
> to return fi_extent_max extents, but actually it won't work for
> a sparse file. The reason is that in xfs_getbmap we will
> calculate holes and set it in 'out', while out is malloced by
> bmv_count(fi_extent_max+1) which didn't consider holes. So in the
> worst case, if 'out' vector looks like
> [hole, extent, hole, extent, hole, ... hole, extent, hole],
> we will only return half of fi_extent_max extents.
> 
> So in xfs_vn_fiemap, we should consider this worst case. If the
> user wants fi_extent_max extents, we need a 'out' with size of
> 2 *fi_extent_max + 1.

This all seems right to me, though your commit message above (+1)
doesn't match the comment and code in the patch (+2)

-Eric

> Cc: Alex Elder <aelder@xxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Dave Chinner <david@xxxxxxxxxxxxx>
> Signed-off-by: Tao Ma <tao.ma@xxxxxxxxxx>
> ---
>  fs/xfs/linux-2.6/xfs_iops.c |   16 ++++++++++++++--
>  1 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/linux-2.6/xfs_iops.c b/fs/xfs/linux-2.6/xfs_iops.c
> index 9c8019c..1db92e3 100644
> --- a/fs/xfs/linux-2.6/xfs_iops.c
> +++ b/fs/xfs/linux-2.6/xfs_iops.c
> @@ -672,9 +672,21 @@ xfs_vn_fiemap(
>  	else
>  		bm.bmv_length = BTOBB(length);
>  
> -	/* We add one because in getbmap world count includes the header */
> +	/*
> +	 * It is a bit tricky for us to calculate the bmv_count from
> +	 * fi_extent_max.
> +	 * If we support to return fi_extent_max extents to the user,
> +	 * we need at most 2 * fi_extent_max + 1 for bmv_count since
> +	 * in xfs_getbmap we will calculate holes while fi_extent_max
> +	 * don't have them. So in the worst case, bmv can looks like
> +	 * [hole, extent, hole, extent, hole, ... hole, extent, hole].
> +	 * So there will be 2 *fi_extent_max + 1.
> +	 * What's more, in getbmap world count have to include the
> +	 * header, so we need another bmv. So the total number will
> +	 * be 2 * fieinfo->fi_extents_max + 2.
> +	 */
>  	bm.bmv_count = !fieinfo->fi_extents_max ? MAXEXTNUM :
> -					fieinfo->fi_extents_max + 1;
> +			2 * fieinfo->fi_extents_max + 2;
>  	bm.bmv_count = min_t(__s32, bm.bmv_count,
>  			     (PAGE_SIZE * 16 / sizeof(struct getbmapx)));
>  	bm.bmv_iflags = BMV_IF_PREALLOC;

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