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

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Date:	Fri, 18 Jun 2010 16:22:25 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Tao Ma <tao.ma@...cle.com>
Cc:	xfs@....sgi.com, linux-kernel@...r.kernel.org,
	Alex Elder <aelder@....com>, Christoph Hellwig <hch@....de>,
	Eric Sandeen <sandeen@...hat.com>
Subject: Re: [PATCH v2] xfs: Make fiemap works with sparse file.

On Fri, Jun 18, 2010 at 10:27:49AM +0800, Tao Ma wrote:
> >>This patch add a new parameter BMV_IF_NO_HOLES for bvm_iflags.
> >>So with this flags, we don't use our 'out' in xfs_getbmap for
> >>a hole. The solution is a bit ugly by just don't increasing
> >>index of 'out' vector. I felt that it is not easy to skip it
> >>at the very beginning since we have the complicated check and
> >>some function like xfs_getbmapx_fix_eof_hole to adjust 'out'.
> >
> >... because I think we can safely skip xfs_getbmapx_fix_eof_hole()
> >it only modifies the hole. Hence  just adding a check after the
> >attribute fork end check (which needs to detect a hole to terminate)
> >should be fine: e.g something like:
> >
> >			if (map[i].br_startblock == HOLESTARTBLOCK&&
> >			whichfork == XFS_ATTR_FORK) {
> >				/* came to the end of attribute fork */
> >				out[cur_ext].bmv_oflags |= BMV_OF_LAST;
> >				goto out_free_map;
> >			}
> >+			if (map[i].br_startblock == HOLESTARTBLOCK&&
> >+				 (iflags&  BMV_IF_NO_HOLES)) {
> >+				memset(&out[cur_ext], 0, sizeof(out[cur_ext]));
> >+				continue;
> >+			}
> >
> >Should work and avoid the worst of the ugliness.
> I am afraid it doesn't work, at least from my test. It enters a dead loop.
> I think the root cause is that your change doesn't update bmv_offset
> and bmv_length for a hole. So in the large loop,

Yes, you are right - I didn't consider the outer loop termination
properly. My bad.

I'll add your patch to my qa list and move it onwards. Thanks.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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