Messages in this thread | | | Date | Mon, 14 Jun 2010 21:37:18 +0800 | From | Tao Ma <> | Subject | Re: [PATCH v2] xfs: Make fiemap works with sparse file. |
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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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