Description of problem:
TRIM doesn't work on EXT4

Version-Release number of selected component (if applicable):
kernel-2.6.32-279.2.1.el6.x86_64

How reproducible:
Always


/etc/fstab:
UUID=ea113042-48a8-4ca7-9b12-2d147ec0adf9  /            ext4    defaults,noatime,nodiratime,discard        1 1

# hdparm -I /dev/sda | grep -i trim
* Data Set Management TRIM supported (limit 8 blocks)

# smartctl -i /dev/sda | grep Model
Device Model:     Corsair Performance Pro


TEST TRIM:
1. Create test file

# seq 1 1000 > testfile
# sync

2. Read sector

# hdparm --fibmap testfile
 filesystem blocksize 4096, begins at LBA 63; assuming 512 byte sectors.
 byte_offset  begin_LBA    end_LBA    sectors
           0   46403743   46403750          8

# hdparm --read-sector 46403743 /dev/sda
reading sector 46403743: succeeded
0a31 0a32 0a33 0a34 0a35 0a36 0a37 0a38
...

3. Remove file

# rm -f testfile
# sync
# sleep 10

4. Verify TRIM:
# hdparm --read-sector 46403743 /dev/sda

/dev/sda:
reading sector 46403743: succeeded
0a31 0a32 0a33 0a34 0a35 0a36 0a37 0a38
...

Result: TRIM doesn't work. All values should be zeros:
reading sector 46403743: succeeded
0000 0000 0000 0000 0000 0000 0000 0000
...


fstrim doesn't TRIM data too (values are not zeros).

# fstrim -v /
/: 90123654612 bytes were trimmed

# fstrim -v /
/: 0 bytes were trimmed



Test TRIM howtos with screenshots:
* http://nedoboi.wordpress.com/2011/11/12/tiny-tips-ssd-and-linux-enable-trim-and-check-if-it-works/
* http://techgage.com/article/enabling_and_testing_ssd_trim_support_under_linux/2

Has anybody had this problem or TRIM clean data as expected?

kernel-ml-3.5.0-2.el6.elrepo.x86_64 from ELrepo doesn't fix the problem.