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Date: Mon, 14 Jun 2010 10:27:06 +1000 From: Dave Chinner <david@...morbit.com> To: Tao Ma <tao.ma@...cle.com> Cc: xfs@....sgi.com, linux-kernel@...r.kernel.org, sandeen@...deen.net, Alex Elder <aelder@....com>, Christoph Hellwig <hch@....de> Subject: Re: [PATCH v2] xfs: Make fiemap works with sparse file. On Sat, Jun 12, 2010 at 10:08:15AM +0800, 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. Define "won't work". i.e. what's the test case? I just created a sparse file and checked it, and it reported all the extents in it: # xfs_bmap -vp testfile testfile: EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL FLAGS 0: [0..7]: hole 8 1: [8..15]: 96..103 0 (96..103) 8 00000 2: [16..23]: hole 8 3: [24..31]: 112..119 0 (112..119) 8 00000 4: [32..39]: hole 8 5: [40..47]: 128..135 0 (128..135) 8 00000 6: [48..55]: hole 8 7: [56..63]: 144..151 0 (144..151) 8 00000 8: [64..71]: hole 8 9: [72..79]: 160..167 0 (160..167) 8 00000 10: [80..87]: hole 8 11: [88..95]: 176..183 0 (176..183) 8 00000 12: [96..103]: hole 8 13: [104..111]: 192..199 0 (192..199) 8 00000 14: [112..119]: hole 8 15: [120..127]: 208..215 0 (208..215) 8 00000 # filefrag -v testfile Filesystem type is: 58465342 File size of testfile is 65536 (16 blocks, blocksize 4096) ext logical physical expected length flags 0 1 12 1 1 3 14 12 1 2 5 16 14 1 3 7 18 16 1 4 9 20 18 1 5 11 22 20 1 6 13 24 22 1 7 15 26 24 1 eof testfile: 9 extents found # FWIW, filefrag seems busted - the file has 8 extents, not 9. For a more fragmented sparse file (25,000 extents): # for i in `seq 1 2 50000`; do dd if=/dev/zero of=testfile bs=4k count=1 seek=$i; done .... # xfs_bmap -vp testfile | grep -v hole | wc -l 25002 # filefrag -v testfile |tail -1 testfile: 25001 extents found So taking away the 2 header lines from xfs_bmap output we have 25000 extents, and filefrag has over-counted by one again. However, we are we are definitely finding all the extents through fiemap... > 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.... Cheers, Dave. -- Dave Chinner david@...morbit.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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