Re: [Qemu-devel] [Bug 1179219] Re: segfault in alloc_refcount_block

[Brian J. Murrell]

On 13-05-15 04:45 AM, Stefan Hajnoczi wrote:
> 
> $ qemu-img create -f qcow2 -b template.img vm001.qcow2
> $ qemu-img create -f qcow2 -b template.img vm002.qcow2
> $ qemu-img create -f qcow2 -b template.img vm003.qcow2
> $ ...launch vm001-vm002...
> 
> This is safe.

Yes.  This is exactly what I did.

> The template.img backing file is open read-only.

That was my understanding also.

> Did you do anything else?

No, not other than just run the VMs on the snapshot volumes.

In any case, I have moved my host from EL6 to Fedora 18 and things seem
more stable, at least in these early days.  :-)

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https://bugs.launchpad.net/bugs/1179219

Title:
  segfault in  alloc_refcount_block

Status in QEMU:
  Invalid

Bug description:
  On CentOS-6.4.latest, I am trying to run several KVM VMs with
  snapshots of a single qcow2 image.  Randomly some VMs will crash
  though.  There's a downstream bug report at
  http://bugs.centos.org/view.php?id=6441 and included in that is an
  "abrt" crash report that contains the full stack trace as well as
  disassembly etc.  That report is at
  http://bugs.centos.org/file_download.php?file_id=1486&type=bug

  For convenience I will paste the segfaulting thread's stack trace
  here:

  :#0  0x00007f0d4d9fadd5 in alloc_refcount_block (bs=0x7f0d4fc38010, 
offset=864752701576067072, length=<value optimized out>, addend=-1) at 
/usr/src/debug/qemu-kvm-0.12.1.2/block/qcow2-refcount.c:335
  :        refcount_table_index = 402681856
  :        new_block = 131072
  :        table_size = <value optimized out>
  :        new_table = <value optimized out>
  :        old_table_offset = <value optimized out>
  :        old_free_cluster_index = <value optimized out>
  :        last_table_size = <value optimized out>
  :        refcount_block_clusters = <value optimized out>
  :        meta_offset = 2147483648
  :        table_offset = 2147614720
  :        s = 0x10000
  :        blocks_used = 1
  :        old_table_size = <value optimized out>
  :        ret = <value optimized out>
  :        new_blocks = 0x7f0d504babd0
  :        i = <value optimized out>
  :        table_clusters = <value optimized out>
  :        data = "\000\000\000\000\000\000\000\000e\240Y\003"
  :        blocks_clusters = <value optimized out>
  :        block = <value optimized out>
  :#1  update_refcount (bs=0x7f0d4fc38010, offset=864752701576067072, 
length=<value optimized out>, addend=-1) at 
/usr/src/debug/qemu-kvm-0.12.1.2/block/qcow2-refcount.c:460
  :        block_index = <value optimized out>
  :        refcount = <value optimized out>
  :        cluster_index = 13195079064576
  :        table_index = 402681856
  :        s = 0x7f0d4fc35770
  :        start = 864752701576052736
  :        last = 864752701576118272
  :        cluster_offset = 864752701576052736
  :        refcount_block = 0x0
  :        old_table_index = <value optimized out>
  :        ret = <value optimized out>
  :#2  0x00007f0d4d9fb710 in qcow2_free_clusters (bs=0x7f0d4fc38010, 
offset=864752701576067072, size=65536) at 
/usr/src/debug/qemu-kvm-0.12.1.2/block/qcow2-refcount.c:640
  :        ret = <value optimized out>
  :#3  0x00007f0d4d9fd03e in qcow2_alloc_cluster_link_l2 (bs=0x7f0d4fc38010, 
m=<value optimized out>) at 
/usr/src/debug/qemu-kvm-0.12.1.2/block/qcow2-cluster.c:674
  :        s = <value optimized out>
  :        i = <value optimized out>
  :        j = <value optimized out>
  :        l2_index = 2032
  :        ret = <value optimized out>
  :        old_cluster = 0x7f0d4fd2b5e0
  :        start_sect = <value optimized out>
  :        l2_offset = 145358848
  :        l2_table = 0x0
  :        cluster_offset = <value optimized out>
  :        cow = <value optimized out>
  :#4  0x00007f0d4d9f7d39 in qcow2_co_writev (bs=0x7f0d4fc38010, 
sector_num=<value optimized out>, remaining_sectors=216, qiov=0x7f0d40051b40) 
at /usr/src/debug/qemu-kvm-0.12.1.2/block/qcow2.c:632
  :        s = 0x7f0d4fc35770
  :        index_in_cluster = 120
  :        n_end = <value optimized out>
  :        ret = <value optimized out>
  :        cur_nr_sectors = 8
  :        cluster_offset = 274333696
  :        hd_qiov = {iov = 0x7f0d4001bcb0, niov = 1, nalloc = 26, size = 4096}
  :        bytes_done = <value optimized out>
  :        cluster_data = 0x0
  :        l2meta = {offset = 2817585152, cluster_offset = 274333696, n_start = 
120, nb_available = 128, nb_clusters = 1, depends_on = 0x0, dependent_requests 
= {entries = {tqh_first = 0x0, tqh_last = 0x7f0cf43dde78}}, next_in_flight = 
{le_next = 0x0, le_prev = 0x7f0cefffee88}}
  :        __PRETTY_FUNCTION__ = "qcow2_co_writev"
  :#5  0x00007f0d4d9e20b9 in bdrv_co_do_writev (bs=0x7f0d4fc38010, 
sector_num=5503096, nb_sectors=216, qiov=0x7f0d40051b40, flags=<value optimized 
out>) at /usr/src/debug/qemu-kvm-0.12.1.2/block.c:2081
  :        drv = 0x7f0d4de96f80
  :        req = {bs = 0x7f0d4fc38010, sector_num = 5503096, nb_sectors = 216, 
is_write = true, list = {le_next = 0x0, le_prev = 0x7f0cefffef28}, co = 
0x7f0d40002af0, wait_queue = {entries = {tqh_first = 0x0, tqh_last = 
0x7f0cf43ddf40}}}
  :        ret = <value optimized out>
  :#6  0x00007f0d4d9e2161 in bdrv_co_do_rw (opaque=0x7f0d4003ced0) at 
/usr/src/debug/qemu-kvm-0.12.1.2/block.c:3497
  :        acb = 0x7f0d4003ced0
  :        bs = <value optimized out>
  :#7  0x00007f0d4d9e7eeb in coroutine_trampoline (i0=<value optimized out>, 
i1=<value optimized out>) at 
/usr/src/debug/qemu-kvm-0.12.1.2/coroutine-ucontext.c:129
  :        self = 0x7f0d40002af0
  :        co = 0x7f0d40002af0
  :#8  0x00007f0d4b31bb70 in ?? () from /lib64/libc-2.12.so
  :No symbol table info available.
  :#9  0x00007f0d44c0eed0 in ?? ()
  :No symbol table info available.
  :#10 0x0000000000000000 in ?? ()

  It would appear, according to RPM at least that I am using 0.12.1.2 of
  qemu/kvm on this machine.

  I'm happy to provide any additional information test patches, etc.

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