Re: [PATCH] filefrag: fix wrong extent count calculation when using FIBMAP

[Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>]

Hi,

On 10/07/2014 06:29 AM, Andreas Dilger wrote:
> On Oct 6, 2014, at 5:05 AM, Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com> wrote:
>> When using FIBMAP and '-e' option is specified, the calculation for
>> fiemap_extent is wrong, we wrongly updated fm_ext.fe_logical for every iteration, please see the code in the end of 'for' loop in
>> fm_ext.fe_logical().
>>
>> In an ext2 file system(block size is 1024 bytes),
>>  dd if=/dev/zero of=testfile bs=1k count=15
>>  Using debugfs, corresponding physical blocks are "2049 2050 2051 2052 2053 2054
>> 2055 2056 2057 2058 2059 2060 1025 2061 2062 2063",  1025 is this indirect block.
>> Before this patch, filefrag's output would be:
>>  filefrag -B -e testfile
>>  Filesystem type is: ef53
>>  Filesystem cylinder groups approximately 16
>>  File size of testfile is 15360 (15 blocks of 1024 bytes)
>>   ext:     logical_offset:        physical_offset: length:   expected: flags:
>>     0:        1..       2:       2050..      2051:      2:       2051: merged
>>     1:        3..       4:       2052..      2053:      2:       2053: merged
>>     2:        5..       6:       2054..      2055:      2:       2055: merged
>>     3:        7..       8:       2056..      2057:      2:       2057: merged
>>     4:        9..      10:       2058..      2059:      2:       2059: merged
>>     5:       11..      12:       2060..      2061:      2:       2062: merged
>>     6:       13..      14:       2062..      2063:      2:       2063: merged,eof
>>     7:       14..      14:       2063..      2063:      1:       2063: merged,eof
>> This output is not reasonable.
> 
> Definitely agree.
> 
>> Fix this bug and try to make it readable. After this patch, the output would be:
>>  ./filefrag -B -e mntpoint/testfile
>>  Filesystem type is: ef53
>>  Filesystem cylinder groups approximately 16
>>  File size of mntpoint/testfile is 15360 (15 blocks of 1024 bytes)
>>   ext:     logical_offset:        physical_offset: length:   expected: flags:
>>     0:        0..      11:       2049..      2060:     12:       2062: merged
>>     1:       12..      14:       2061..      2063:      3:       2063: merged,eof
>>  mntpoint/testfile: 2 extents found, perfection would be 1 extent
> 
> Why would this not be shown as a single extent:
> 
>     0:      0..14:       2049..2063:    15:          merged,eof
> 
> It isn't clear why "expected" is 2062 instead of 2061.

This is because for a logical region, only all corresponding physical data blocks and
possible indirect blocks are both continuous, we can say that it's an extent. There is such
code in filefrag_fibmap():
if (is_ext2 && last_block) {
	if (((i - EXT2_DIRECT) % bpib) == 0)
		last_block++;
	if (((i - EXT2_DIRECT - bpib) % (bpib * bpib)) == 0)
		last_block++;
	if (((i - EXT2_DIRECT - bpib - bpib * bpib) %
	     (((unsigned long long)bpib) * bpib * bpib)) == 0)
		last_block++;
}

For logical block 12, it starts to need an indirect block, and its expected physical
block is 2062(expected indirect block is 2061), so we do not consider it continuous.

Regards,
Xiaoguang Wang
> 
> Cheers, Andreas
> 
>> Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
>> ---
>> misc/filefrag.c | 37 +++++++++++++++++++++++--------------
>> 1 file changed, 23 insertions(+), 14 deletions(-)
>>
>> diff --git a/misc/filefrag.c b/misc/filefrag.c
>> index c1a8684..e9f7e68 100644
>> --- a/misc/filefrag.c
>> +++ b/misc/filefrag.c
>> @@ -315,32 +315,41 @@ static int filefrag_fibmap(int fd, int blk_shift, int *num_extents,
>> 			return rc;
>> 		if (block == 0)
>> 			continue;
>> +		count++;
>> +
>> 		if (*num_extents == 0) {
>> 			(*num_extents)++;
>> 			if (force_extent) {
>> 				print_extent_header();
>> +				fm_ext.fe_logical = logical;
>> 				fm_ext.fe_physical = block * st->st_blksize;
>> +				fm_ext.fe_length = st->st_blksize;
>> 			}
>> +			last_block = block;
>> +			continue;
>> 		}
>> -		count++;
>> -		if (force_extent && last_block != 0 &&
>> -		    (block != last_block + 1 ||
>> -		     fm_ext.fe_logical + fm_ext.fe_length != logical)) {
>> -			print_extent_info(&fm_ext, *num_extents - 1,
>> -					  (last_block + 1) * st->st_blksize,
>> -					  blk_shift, st);
>> -			fm_ext.fe_length = 0;
>> -			(*num_extents)++;
>> +
>> +		if (force_extent) {
>> +			if (block != last_block + 1 ||
>> +			    fm_ext.fe_length + fm_ext.fe_logical != logical) {
>> +				print_extent_info(&fm_ext, *num_extents - 1,
>> +						  (last_block + 1) *
>> +						  st->st_blksize,
>> +						  blk_shift, st);
>> +				fm_ext.fe_logical = logical;
>> +				fm_ext.fe_physical = block * st->st_blksize;
>> +				fm_ext.fe_length = st->st_blksize;
>> +				(*num_extents)++;
>> +			} else {
>> +				fm_ext.fe_length += st->st_blksize;
>> +			}
>> 		} else if (last_block && (block != last_block + 1)) {
>> -			if (verbose)
>> +			if (verbose) {
>> 				printf("Discontinuity: Block %ld is at %lu (was "
>> 				       "%lu)\n", i, block, last_block + 1);
>> -			fm_ext.fe_length = 0;
>> +			}
>> 			(*num_extents)++;
>> 		}
>> -		fm_ext.fe_logical = logical;
>> -		fm_ext.fe_physical = block * st->st_blksize;
>> -		fm_ext.fe_length += st->st_blksize;
>> 		last_block = block;
>> 	}
>>
>> -- 
>> 1.8.2.1
>>
>> --
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> 
> 
> Cheers, Andreas
> 
> 
> 
> 
>