Yesterday one of my Xen DomU's died with:

Unable to handle kernel paging request at ffff8800064d9000 RIP:
[<ffffffff801b99c7>] leaf_copy_items_entirely+0x15f/0x1fb
PGD 516067 PUD 517067 PMD 54a067 PTE 80100000064d9065
Oops: 0003 [1]
CPU 0
Modules linked in:
Pid: 25320, comm: sshd Not tainted 2.6.21-xen #1
RIP: e030:[<ffffffff801b99c7>] [<ffffffff801b99c7>] leaf_copy_items_entirely+0x15f/0x1fb
RSP: e02b:ffff88000620f228 EFLAGS: 00210282
RAX: 0000000000000b64 RBX: 000000000000fc3c RCX: fffffffffffc60d8
RDX: ffff880003ff8af0 RSI: ffff8800009eaf28 RDI: ffff8800064d9000
RBP: ffff8800064aec3c R08: ffff8800064aec3c R09: 0000000000000000
R10: 0000000000000120 R11: ffff8800049b0000 R12: 0000000000000210
R13: 00000000000000d8 R14: 0000000000000008 R15: ffff88000649f000
FS: 0000000000000000(0000) GS:ffffffff80434000(0063) knlGS:0000000000000000
CS: e033 DS: 002b ES: 002b CR0: 000000008005003b
CR2: 0000000000000000 CR3: 000000000278a000 CR4: 0000000000000660
Process sshd (pid: 25320, threadinfo ffff88000620e000, task ffff880002494990)
Stack: 0000000000000016 ffff880006223d60 ffff88000620f2c8 ffff88000649f0c0
fdf088000620f2c8 ffff880003ff8af0 0000000803ff8af0 0000000000000016
0000000000000000 ffff880006223d60 ffff88000620f2c8 0000000000000016
Call Trace:
[<ffffffff801bb509>] leaf_move_items+0x21f/0x321
[<ffffffff801c19b1>] get_cnode+0x5c/0x61
[<ffffffff801bb683>] leaf_shift_left+0x30/0x95
[<ffffffff801a6e49>] do_balance+0x420/0x271b
[<ffffffff80184dd4>] __getblk+0x1b/0x1c7
[<ffffffff801861b9>] __bread+0x6/0x74
[<ffffffff801c0b45>] reiserfs_prepare_for_journal+0x4f/0x56
[<ffffffff801b4e8d>] fix_nodes+0x501/0x7c6
[<ffffffff801bed82>] reiserfs_delete_item+0x311/0x34e
[<ffffffff801c0725>] direct2indirect+0x34c/0x3fb
[<ffffffff801ad8ff>] reiserfs_get_block+0x9c3/0xf70
[<ffffffff80185482>] ll_rw_block+0x8b/0xa4
[<ffffffff8018560e>] __block_prepare_write+0x173/0x42a
[<ffffffff801acf3c>] reiserfs_get_block+0x0/0xf70
[<ffffffff801858df>] block_prepare_write+0x1a/0x24
[<ffffffff801abfa8>] reiserfs_prepare_write+0xc9/0x126
[<ffffffff80184915>] __generic_cont_expand+0x9f/0xfa
[<ffffffff801b0667>] reiserfs_file_write+0x2e3/0x18e8
[<ffffffff801b5d48>] reiserfs_dirty_inode+0x61/0x68
[<ffffffff80127dfa>] current_fs_time+0x35/0x3a
[<ffffffff80181b0f>] __mark_inode_dirty+0x2f/0x171
[<ffffffff8017a372>] touch_atime+0xbe/0x104
[<ffffffff801489f6>] do_generic_mapping_read+0x3c1/0x407
[<ffffffff80147e55>] file_read_actor+0x0/0x118
[<ffffffff80176316>] __posix_lock_file_conf+0x35f/0x394
[<ffffffff80176db5>] fcntl_setlk+0x214/0x241
[<ffffffff80169a80>] vfs_write+0xad/0x136
[<ffffffff80169f3c>] sys_write+0x45/0x6e
[<ffffffff80117eb8>] ia32_sysret+0x0/0xa
[<ffffffff80117e40>] ia32_syscall+0x0/0x5a

Code: f3 a4 8b 44 24 34 03 04 24 66 41 89 47 02 66 41 29 dd 44 89
RIP [<ffffffff801b99c7>] leaf_copy_items_entirely+0x15f/0x1fb
RSP <ffff88000620f228>
CR2: ffff8800064d9000
BUG: at /usr/src/linux-2.6.21-xen/kernel/exit.c:861 do_exit()

Call Trace:
[<ffffffff8012651e>] do_exit+0x47/0x790
[<ffffffff801207cd>] __wake_up+0x23/0x46
[<ffffffff801145f4>] do_page_fault+0x1206/0x1282
[<ffffffff80108c94>] __switch_to+0x351/0x45d
[<ffffffff80346eb7>] error_exit+0x0/0x69
[<ffffffff801b99c7>] leaf_copy_items_entirely+0x15f/0x1fb
[<ffffffff801b9994>] leaf_copy_items_entirely+0x12c/0x1fb
[<ffffffff801bb509>] leaf_move_items+0x21f/0x321
[<ffffffff801c19b1>] get_cnode+0x5c/0x61
[<ffffffff801bb683>] leaf_shift_left+0x30/0x95
[<ffffffff801a6e49>] do_balance+0x420/0x271b
[<ffffffff80184dd4>] __getblk+0x1b/0x1c7
[<ffffffff801861b9>] __bread+0x6/0x74
[<ffffffff801c0b45>] reiserfs_prepare_for_journal+0x4f/0x56
[<ffffffff801b4e8d>] fix_nodes+0x501/0x7c6
[<ffffffff801bed82>] reiserfs_delete_item+0x311/0x34e
[<ffffffff801c0725>] direct2indirect+0x34c/0x3fb
[<ffffffff801ad8ff>] reiserfs_get_block+0x9c3/0xf70
[<ffffffff80185482>] ll_rw_block+0x8b/0xa4
[<ffffffff8018560e>] __block_prepare_write+0x173/0x42a
[<ffffffff801acf3c>] reiserfs_get_block+0x0/0xf70
[<ffffffff801858df>] block_prepare_write+0x1a/0x24
[<ffffffff801abfa8>] reiserfs_prepare_write+0xc9/0x126
[<ffffffff80184915>] __generic_cont_expand+0x9f/0xfa
[<ffffffff801b0667>] reiserfs_file_write+0x2e3/0x18e8
[<ffffffff801b5d48>] reiserfs_dirty_inode+0x61/0x68
[<ffffffff80127dfa>] current_fs_time+0x35/0x3a
[<ffffffff80181b0f>] __mark_inode_dirty+0x2f/0x171
[<ffffffff8017a372>] touch_atime+0xbe/0x104
[<ffffffff801489f6>] do_generic_mapping_read+0x3c1/0x407
[<ffffffff80147e55>] file_read_actor+0x0/0x118
[<ffffffff80176316>] __posix_lock_file_conf+0x35f/0x394
[<ffffffff80176db5>] fcntl_setlk+0x214/0x241
[<ffffffff80169a80>] vfs_write+0xad/0x136
[<ffffffff80169f3c>] sys_write+0x45/0x6e
[<ffffffff80117eb8>] ia32_sysret+0x0/0xa
[<ffffffff80117e40>] ia32_syscall+0x0/0x5a

Which caused all kinds of problems and eventually causing the DomU to die until I ran fsck. The closest thing I've found is the Crash DomU for reiserfs filesystem bug un Launchpad. Unfortunately it's closed and not fixed. I'm a bit puzzled wether this is caused by bad hardware or it's a real bug...