[Bug 593086] Re: Silent wraparound on > 2 TB LVM snapshots

[Phillip Susi]

After discussing it on dm-devel at redhat.com, I believe the issue has been
found.  There was a patch in 2.6.32 that caused this breakage, and was
later fixed.  The broken patch ended up in Ubuntu Lucid and Karmic
kernels.  This probably should be corrected with an SRU.  From the
mailing list discussion Mikulas Patocka wrote:

The bug existed even in upstream, but only in 2.6.32 kernel. The reason 
was this function:
static inline chunk_t sector_to_chunk(struct dm_exception_store *store,
                                      sector_t sector)
{
        return (sector & ~store->chunk_mask) >> store->chunk_shift;
}

"store->chunk_mask" was changed to be unsigned in 2.6.32, so it was 
masking the sector with 32-bit value. In 2.6.33 that masking was removed. 
Ubuntu picked that 2.6.32 patch but didn't pick further patches.


** Description changed:

  Ubuntu kernels silently wrap access to disk locations above 2 TB back
  around to zero.  This can easily be reproduced using lvm to create a
  thin provisioned virtual disk:
  
  lvcreate -s -n thin -L 1g --virtualsize 3t vg0
  
  mke2fs -t ext4 -E lazy_itable_init /dev/vg0/thin
  e2fsck -f /dev/vg0/thin
  
  Fsck will find errors in the bitmap because it is actually reading the
  superblock instead of the allocation bitmap situated just after the 2 tb
  mark.
- 
- 
- --- 
- AlsaVersion: Advanced Linux Sound Architecture Driver Version 1.0.21.
- Architecture: amd64
- AudioDevicesInUse:
-  USER        PID ACCESS COMMAND
-  /dev/snd/controlC0:  psusi      1874 F.... pulseaudio
- CRDA: Error: [Errno 2] No such file or directory
- Card0.Amixer.info:
-  Card hw:0 'NVidia'/'HDA NVidia at 0xfe024000 irq 20'
-    Mixer name	: 'Realtek ALC888'
-    Components	: 'HDA:10ec0888,1565820c,00100001'
-    Controls      : 37
-    Simple ctrls  : 20
- Card1.Amixer.info:
-  Card hw:1 'HDMI'/'HDA ATI HDMI at 0xfd9fc000 irq 16'
-    Mixer name	: 'ATI R6xx HDMI'
-    Components	: 'HDA:1002aa01,00aa0100,00100100'
-    Controls      : 4
-    Simple ctrls  : 1
- Card1.Amixer.values:
-  Simple mixer control 'IEC958',0
-    Capabilities: pswitch pswitch-joined penum
-    Playback channels: Mono
-    Mono: Playback [off]
- DistroRelease: Ubuntu 10.04
- InstallationMedia: Ubuntu 9.10 "Karmic Koala" - Release amd64 (20091027)
- IwConfig:
-  lo        no wireless extensions.
-  
-  eth0      no wireless extensions.
- Lsusb:
-  Bus 002 Device 003: ID 045e:0039 Microsoft Corp. IntelliMouse Optical
-  Bus 002 Device 002: ID 045e:00db Microsoft Corp. Natural Ergonomic Keyboard 4000 V1.0
-  Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
-  Bus 001 Device 004: ID 058f:6387 Alcor Micro Corp. Transcend JetFlash Flash Drive
-  Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
- MachineType: BIOSTAR Group TF560 A2+
- Package: linux (not installed)
- ProcCmdLine: BOOT_IMAGE=/boot/vmlinuz-2.6.32-22-generic root=/dev/mapper/hostname-lucid ro quiet splash acpi_enforce_resources=lax
- ProcEnviron:
-  LANG=en_US.utf8
-  SHELL=/bin/bash
- ProcVersionSignature: Ubuntu 2.6.32-22.36-generic 2.6.32.11+drm33.2
- Regression: No
- RelatedPackageVersions: linux-firmware 1.34
- Reproducible: Yes
- RfKill:
-  
- Tags: lucid  needs-upstream-testing
- Uname: Linux 2.6.32-22-generic x86_64
- UserGroups: adm admin cdrom dialout dip fax floppy fuse lpadmin plugdev sambashare tape video
- WifiSyslog: Jun 14 22:36:08 faldara kernel: [ 1446.372143] lo: Disabled Privacy Extensions
- dmi.bios.date: 06/11/2007
- dmi.bios.vendor: Phoenix Technologies, LTD
- dmi.bios.version: 6.00 PG
- dmi.board.name: TF560 A2+
- dmi.board.vendor: BIOSTAR Group
- dmi.board.version: 1.0
- dmi.chassis.type: 3
- dmi.chassis.vendor: BIOSTAR Group
- dmi.chassis.version: TF560 A2+
- dmi.modalias: dmi:bvnPhoenixTechnologies,LTD:bvr6.00PG:bd06/11/2007:svnBIOSTARGroup:pnTF560A2+:pvr1.0:rvnBIOSTARGroup:rnTF560A2+:rvr1.0:cvnBIOSTARGroup:ct3:cvrTF560A2+:
- dmi.product.name: TF560 A2+
- dmi.product.version: 1.0
- dmi.sys.vendor: BIOSTAR Group

** Changed in: linux (Ubuntu)
       Status: Incomplete => Triaged

** Tags removed: apport-collected needs-kernel-logs needs-upstream-
testing

-- 
Silent wraparound on > 2 TB LVM snapshots
https://bugs.launchpad.net/bugs/593086
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