[Bug 1923583] Re: colo: pvm flush failed after svm killed

[Thomas Huth]

** Changed in: qemu
       Status: Fix Committed => Fix Released

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Title:
  colo: pvm flush failed after svm killed

Status in QEMU:
  Fix Released

Bug description:
  Hi,
     Primary vm flush failed after killing svm, which leads primary vm guest 
filesystem unavailable.

  qemu versoin: 5.2.0
  host/guest os: CentOS Linux release 7.6.1810 (Core)

  Reproduce steps:
  1. create colo vm following 
https://github.com/qemu/qemu/blob/master/docs/COLO-FT.txt
  2. kill secondary vm (don't remove nbd child from quorum on primary vm)and 
wait for a minute. the interval depends on guest os.
  result: primary vm file system shutdown because of flush cache error.

  After serveral tests, I found that qemu-5.0.0 worked well, and it's
  the commit
  
https://git.qemu.org/?p=qemu.git;a=commit;h=883833e29cb800b4d92b5d4736252f4004885191(block:
  Flush all children in generic code) leads this change, and both
  virtio-blk and ide turned out to be bad.

  I think it's nbd(replication) flush failed leads bdrv_co_flush(quorum_bs) 
failed, here is the call stack.
  #0  bdrv_co_flush (bs=0x56242b3cc0b0=nbd_bs) at ../block/io.c:2856
  #1  0x0000562428b0f399 in bdrv_co_flush (bs=0x56242b3c7e00=replication_bs) at 
../block/io.c:2920
  #2  0x0000562428b0f399 in bdrv_co_flush (bs=0x56242a4ad800=quorum_bs) at 
../block/io.c:2920
  #3  0x0000562428b70d56 in blk_do_flush (blk=0x56242a4ad4a0) at 
../block/block-backend.c:1672
  #4  0x0000562428b70d87 in blk_aio_flush_entry (opaque=0x7fd0980073f0) at 
../block/block-backend.c:1680
  #5  0x0000562428c5f9a7 in coroutine_trampoline (i0=-1409269904, i1=32721) at 
../util/coroutine-ucontext.c:173

  While i am not sure whether i use colo inproperly? Can we assume that
  nbd child of quorum immediately removed right after svm crashed? Or
  it's really a bug? Does the following patch fix? Help is needed!
  Thanks a lot！

  diff --git a/block/quorum.c b/block/quorum.c
  index cfc1436..f2c0805 100644
  --- a/block/quorum.c
  +++ b/block/quorum.c
  @@ -1279,7 +1279,7 @@ static BlockDriver bdrv_quorum = {
       .bdrv_dirname                       = quorum_dirname,
       .bdrv_co_block_status               = quorum_co_block_status,
   
  -    .bdrv_co_flush_to_disk              = quorum_co_flush,
  +    .bdrv_co_flush                      = quorum_co_flush,
   
       .bdrv_getlength                     = quorum_getlength,

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