Re: [PATCH v2] xfs: Make fiemap works with sparse file.

[Tao Ma]

Hi Dave,

Dave Chinner wrote:
> On Mon, Jun 14, 2010 at 10:27:06AM +1000, Dave Chinner wrote:
>   
> > On Sat, Jun 12, 2010 at 10:08:15AM +0800, Tao Ma wrote:
> >     
> > > 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.
> > >       
> > Right, it's not broken, we simply return less than fi_extent_mex
> > extents when there are holes. I don't see that as a problem as
> > applications have to handle that case anyway, and....
> >
> >     
> > > 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 + 2(one more the header).
> > >       
> > That's rather dangerous, I think. It relies on other code to catch
> > the buffer overrun that this sets up for fragmented, non-sparse
> > files. Personally I'd much prefer to return fewer extents for sparse
> > files than to add a landmine like this into the kernel code....
> >     
>
> I just had a thought - if you want to avoid holes being reported to
> fiemap, then add a BMV_IF_NO_HOLES flag to xfs_getbmap() and skip
> holes in the mappin gloop when this flag is set. That will make
> fiemap fill in the full number of extents without hacking the
> extent count...
>   
yeah, that should work and I will try to generate a patch for it.
I am not quite familiar with xfs, so please be kind to me if I make some 
stupid mistake in the patch. ;)

Regards,
Tao
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