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    SubjectRe: [BUG 3.0.0-rc1] ksm: NULL pointer dereference in ksm_do_scan()
    * Andrea Arcangeli (aarcange@redhat.com) wrote:
    > On Thu, Jun 02, 2011 at 07:31:43AM -0700, Chris Wright wrote:
    > > * CAI Qian (caiqian@redhat.com) wrote:
    > > > madvise(0x2210000, 4096, 0xc /* MADV_??? */) = 0
    > > > --- SIGSEGV (Segmentation fault) @ 0 (0) ---
    > >
    > > Right, that's just what the program is trying to do, segfault.
    > >
    > > > +++ killed by SIGSEGV (core dumped) +++
    > > > Segmentation fault (core dumped)
    > > >
    > > > Did I miss anything?
    > >
    > > I found it works but not 100% of the time.
    > >
    > > So I just run the bug in a loop.
    >
    > echo 0 >scan_millisecs helps.

    BTW, here's my stack trace (I dropped back to 2.6.39 just to see if it
    happened to be recent regression). It looks like mm_slot is off the list:

    R10: dead000000200200 R11: dead000000100100

    w/ ->mm == NULL. Smells like use after free, but doesn't quite all add up.

    BUG: unable to handle kernel
    ksmd-bug[14824]: segfault at 0 ip 0000000000400677 sp 00007fff987cb8b0 error 6 in ksmd-bug[400000+1000]
    NULL pointer dereference at 0000000000000060
    IP: [<ffffffff815be345>] down_read+0x19/0x28
    PGD 0
    Oops: 0002 [#1] SMP
    last sysfs file: /sys/devices/system/cpu/cpu15/cache/index2/shared_cpu_map
    CPU 6
    Modules linked in: bridge stp vhost_net macvtap macvlan tun kvm_intel kvm ixgbe mdio igb [last unloaded: scsi_wait_scan]

    Pid: 825, comm: ksmd Not tainted 2.6.39+ #23 Supermicro X8DTN/X8DTN
    RIP: 0010:[<ffffffff815be345>] [<ffffffff815be345>] down_read+0x19/0x28
    RSP: 0018:ffff8801b5325e10 EFLAGS: 00010246
    RAX: 0000000000000060 RBX: 0000000000000060 RCX: 00000000ffffffff
    RDX: 0000000000000000 RSI: 0000000000000286 RDI: 0000000000000060
    RBP: ffff8801b5325e20 R08: ffffffffffffffff R09: ffff8801b5325db0
    R10: dead000000200200 R11: dead000000100100 R12: 0000000000000000
    R13: ffffffff81a20e60 R14: 0000000000000000 R15: 0000000000000000
    FS: 0000000000000000(0000) GS:ffff88033fc80000(0000) knlGS:0000000000000000
    CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
    CR2: 0000000000000060 CR3: 0000000001a03000 CR4: 00000000000006e0
    DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
    DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
    Process ksmd (pid: 825, threadinfo ffff8801b5324000, task ffff8801b55196b0)
    Stack:
    ffff8801b5325e20 0000000000000060 ffff8801b5325ee0 ffffffff810eec3a
    ffff88033fd51cc0 ffff8801b5325e80 ffff8801b5325ee0 ffffffff00000000
    ffff8801b55196b0 ffff8801b5325e98 ffff8801b5324000 00000064b5324010
    Call Trace:
    [<ffffffff810eec3a>] ksm_scan_thread+0x12d/0xc47
    [<ffffffff8105a6e7>] ? wake_up_bit+0x2a/0x2a
    [<ffffffff810eeb0d>] ? try_to_merge_with_ksm_page+0x498/0x498
    [<ffffffff8105a25e>] kthread+0x82/0x8a
    [<ffffffff815c6354>] kernel_thread_helper+0x4/0x10
    [<ffffffff8105a1dc>] ? kthread_worker_fn+0x13f/0x13f
    [<ffffffff815c6350>] ? gs_change+0xb/0xb
    Code: 48 0f c1 10 48 85 d2 74 05 e8 98 84 c8 ff 58 5b c9 c3 55 48 89 e5 53 48 83 ec 08 0f 1f 44 00 00 48 89 fb e8 85 f4 ff ff 48 89 d8 <f0> 48 ff 00 79 05 e8 40 84 c8 ff 5a 5b c9 c3 55 48 89 e5 0f 1f
    RIP [<ffffffff815be345>] down_read+0x19/0x28
    RSP <ffff8801b5325e10>
    CR2: 0000000000000060


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