Fix block migration bug

Because of wrong return value of .save_live_pending() in block-migration, migration finishes before the whole disk is transferred. Such situation occures when the migration process is fast enouth, for example when source and dest are on the same host.

It's easy to test this with the following:

bug.sh
=====================================================================
#!/bin/sh

size=$1
addr=$2

rm /tmp/fifo-mig /tmp/a /tmp/b /tmp/sock-mig

./qemu-img create -f qcow2 /tmp/a $size
./qemu-img create -f qcow2 /tmp/b $size

./qemu-io -c "write -P 0x22 $addr 512" /tmp/a

mkfifo /tmp/fifo-mig

./x86_64-softmmu/qemu-system-x86_64 -drive file=/tmp/b,id=disk\
-qmp unix:/tmp/sock-mig,server,nowait\
-incoming "exec: cat /tmp/fifo-mig" &

echo 'migrate -b exec:cat>/tmp/fifo-mig\nquit\n' |\
./x86_64-softmmu/qemu-system-x86_64 -drive file=/tmp/a,id=disk\
-monitor stdio

./scripts/qmp/qmp --path=/tmp/sock-mig quit
sleep 3

echo checking
./qemu-io -c "read -P 0x22 $addr 512" /tmp/b
=====================================================================

For './bug.sh 1G 1M' qemu-io check finishes successfully, but for './bug.sh 1G 1022M' it finishes with 'Pattern verification failed' status.

The following patch fixes this bug.

Vladimir Sementsov-Ogievskiy (1):
block-migration: fix pending() return value

 block-migration.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

1.9.1

Because of wrong return value of .save_live_pending() in block-migration, migration finishes before the whole disk is transferred. Such situation occures when the migration process is fast enouth, for example when source and dest are on the same host.

If in the bulk phase we return something < max_size, we will skip transferring the tail of the device. Currently we have "set pending to BLOCK_SIZE if it is zero" for bulk phase, but there no guarantee, that it will be < max_size.

True approach is to return, for example, max_size+1 when we are in the bulk phase.

---
 block-migration.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index 73cdd07..bf78bd8 100644
@@ -756,8 +756,8 @@ static uint64_t block_save_pending(QEMUFile *f, void *opaque, uint64_t max_size)
                        block_mig_state.read_done * BLOCK_SIZE;
 
     /* Report at least one block pending during bulk phase */
-    if (pending == 0 && !block_mig_state.bulk_completed) {
-        pending = BLOCK_SIZE;
+    if (pending <= max_size && !block_mig_state.bulk_completed) {
+        pending = max_size + BLOCK_SIZE;
     }
     blk_mig_unlock();
     qemu_mutex_unlock_iothread();

1.9.1

s/occures/occurs/

s/enouth/enough/

I'll let someone else review the code proper.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

Hi there.. Can someone review my bug fix? I'll surely fix typos in description after it.

Best regards,
Vladimir

The file you patch has since moved. Suggest a quick rebase. Make sure to cc: your v2 to migration maintainers.

$ scripts/get_maintainer.pl -f migration/block.c 
Kevin Wolf <[email protected]> (supporter:Block)
Stefan Hajnoczi <[email protected]> (supporter:Block)
Juan Quintela <[email protected]> (maintainer:Migration)
Amit Shah <[email protected]> (maintainer:Migration)

v2 is already pushed http://git.qemu.org/?p=qemu.git;a=commit;h=04636dc410b163c2243e66c3813dd4900a50a4ed

Best regards,
Vladimir