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

[Tao Ma]

Eric Sandeen wrote:
> 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)
>   
oh, yes, I will change the commit log and send a v2.
and can I add your ack for this patch?

Regards,
Tao
> -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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