Chandan Rajendra
2014-Jul-15 09:50 UTC
Fiemap inconsistent behaviour when file offset range isn't on a block boundary
All the filesystems created and used below have 4k blocksize. The
"file.bin" file mentioned below is 8k in size and has two 4k
extents. The test program used can be found at http://fpaste.org/118057/.
1. First run (file range being passed is on block boundaries).
,----
| [root@guest0 btrfs]# for f in $(ls -1 /mnt/{btrfs,ext4,xfs}/file.bin) ;
| > do
| > echo "-------------- File: $f -----------";
| > /root/print-fiemap 0 8192 $f;
| > done
| -------------- File: /mnt/btrfs/file.bin -----------
| File range: 0 - 8191.
| Found 2 extents.
| Fiemap information:
| Logical: 0
| Physical: 12582912
| Length: 4096
| Flags:
|
| Logical: 4096
| Physical: 12656640
| Length: 4096
| Flags: FIEMAP_EXTENT_LAST |
|
| -------------- File: /mnt/ext4/file.bin -----------
| File range: 0 - 8191.
| Found 2 extents.
| Fiemap information:
| Logical: 0
| Physical: 135270400
| Length: 4096
| Flags:
|
| Logical: 4096
| Physical: 135278592
| Length: 4096
| Flags: FIEMAP_EXTENT_LAST |
|
| -------------- File: /mnt/xfs/file.bin -----------
| File range: 0 - 8191.
| Found 2 extents.
| Fiemap information:
| Logical: 0
| Physical: 49152
| Length: 4096
| Flags:
|
| Logical: 4096
| Physical: 57344
| Length: 4096
| Flags: FIEMAP_EXTENT_LAST |
`----
2. If the file offset range being passed as input to fiemap ioctl is
not on block boundaries and falls within an extent's range then that
extent is skipped.
,----
| [root@guest0 btrfs]# for f in $(ls -1 /mnt/{btrfs,ext4,xfs}/file.bin) ;
| > do
| > echo "-------------- File: $f -----------";
| > /root/print-fiemap 1 4095 $f;
| > done
| -------------- File: /mnt/btrfs/file.bin -----------
| File range: 1 - 4095.
| Found 0 extents.
|
|
| -------------- File: /mnt/ext4/file.bin -----------
| File range: 1 - 4095.
| Found 1 extents.
| Fiemap information:
| Logical: 0
| Physical: 135270400
| Length: 4096
| Flags:
|
| -------------- File: /mnt/xfs/file.bin -----------
| File range: 1 - 4095.
| Found 2 extents.
| Fiemap information:
| Logical: 0
| Physical: 49152
| Length: 4096
| Flags:
|
| Logical: 4096
| Physical: 57344
| Length: 4096
| Flags: FIEMAP_EXTENT_LAST |
`----
From linux/Documentation/filesystems/fiemap.txt, "fm_start, and
fm_length specify the logical range within the file which the
process would like mappings for. Extents returned mirror those on
disk - that is, the logical offset of the 1st returned extent may
start before fm_start, and the range covered by the last returned
extent may end after fm_length. All offsets and lengths are in
bytes."
So IMHO, the above would mean that all the extents that map the
file range [fm_start, fm_start + fm_length - 1] should be returned
by a fiemap ioctl call (as done by ext4).
In the case of Btrfs, the commit
ea8efc74bd0402b4d5f663d007b4e25fa29ea778 i.e. "Btrfs: make sure not
to return overlapping extents to fiemap", forces the first extent
returned by btrfs_fiemap() to start from fm_start (if fm_start is
greater than the file offset mapped by the containing extent's
first byte). Can somebody please list some example scenarios where
extent_fiemap() ends up returning dupclicate and overlapping
extents?
Also, the commit 4d479cf010d56ec9c54f3099992d039918f1296b
i.e. "Btrfs: sectorsize align offsets in fiemap", rounds up first
byte of the file offset range to the next block. Shouldn't it be
rounded down instead?
XFS lists both the extents even though the first one encompasses the
file range specified in the input.
--
chandan
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs"
in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html