[Qemu-devel] [Bug 1343827] Re: block.c: multiwrite_merge() truncates overlapping requests

[Stefan Hajnoczi]

Thanks for reporting this bug.  I'm writing a test case and fix, will CC
you on the patches.

Please keep in mind that no ordering is guaranteed between requests that
are in-flight at the same time.  Therefore it is unusual to submit
overlapping requests and could indicate a bug in the application.

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https://bugs.launchpad.net/bugs/1343827

Title:
  block.c: multiwrite_merge() truncates overlapping requests

Status in QEMU:
  New

Bug description:
  If the list of requests passed to multiwrite_merge() contains two
  requests where the first is for a range of sectors that is a strict
  subset of the second's, the second request is truncated to end where
  the first starts, so the second half of the second request is lost.

  This is easy to reproduce by running fio against a virtio-blk device
  running on qemu 2.1.0-rc1 with the below fio script. At least with fio
  2.0.13, the randwrite pass will issue overlapping bios to the block
  driver, which the kernel is happy to pass along to qemu:

  [global]
  randrepeat=0
  ioengine=libaio
  iodepth=64
  direct=1
  size=1M
  numjobs=1
  verify_fatal=1
  verify_dump=1

  filename=$dev

  [seqwrite]
  blocksize_range=4k-1M
  rw=write
  verify=crc32c-intel

  [randwrite]
  stonewall
  blocksize_range=4k-1M
  rw=randwrite
  verify=meta

  Here is a naive fix for the problem that simply avoids merging
  problematic requests. I guess a better solution would be to redo
  qemu_iovec_concat() to do the right thing.

  diff -ur old/qemu-2.1.0-rc2/block.c qemu-2.1.0-rc2/block.c
  --- old/qemu-2.1.0-rc2/block.c  2014-07-15 14:49:14.000000000 -0700
  +++ qemu-2.1.0-rc2/block.c      2014-07-17 23:03:14.224169741 -0700
  @@ -4460,7 +4460,9 @@
           int64_t oldreq_last = reqs[outidx].sector + reqs[outidx].nb_sectors;
   
           // Handle exactly sequential writes and overlapping writes.
  -        if (reqs[i].sector <= oldreq_last) {
  +        // If this request ends before the previous one, don't merge.
  +        if (reqs[i].sector <= oldreq_last &&
  +            reqs[i].sector + reqs[i].nb_sectors >= oldreq_last) {
               merge = 1;
           }

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