[Qemu-devel] [PATCH] hostmem: Disable add/del memory during migration

[Yury Kotov]

I found a bug in QEMU 2.12 with adding memory-backend while live migration
thread is running.

But it seems that this bug was implicitly fixed in this commit (QEMU 3.0):
  b895de50: migration: discard non-migratable RAMBlocks

I think it's better to disallow add/del memory backends during migration to
to prevent other possible problems. Anyway, user can't use this memory because
of disabled hotplug/hotunplug devs.

The idea of this commit is the same as that:
  b06424de: migration: Disable hotplug/unplug memory during migration

Backtrace of this bug in QEMU 2.12:
0  find_next_bit (address@hidden, address@hidden, address@hidden) at 
util/bitops.c:46
1  migration_bitmap_find_dirty (rs=0x7f58f80008c0, start=0, rb=0x5557e66e3200) 
at migration/ram.c:816
2  find_dirty_block (again=<synthetic pointer>, pss=<synthetic pointer>, 
rs=0x7f58f80008c0) at migration/ram.c:1243
3  ram_find_and_save_block (address@hidden, address@hidden) at 
migration/ram.c:1592
4  ram_find_and_save_block (last_stage=false, rs=0x7f58f80008c0) at 
migration/ram.c:2335
5  ram_save_iterate (f=0x5557e69f1000, opaque=<optimized out>) at 
migration/ram.c:2338
6  qemu_savevm_state_iterate (f=0x5557e69f1000, postcopy=false) at 
migration/savevm.c:1191
7  migration_iteration_run (s=0x5557e666b030) at migration/migration.c:2301
8  migration_thread (opaque=0x5557e666b030) at migration/migration.c:2409
9  start_thread (arg=0x7f59055d5700) at pthread_create.c:333
10 clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109

Signed-off-by: Yury Kotov <address@hidden>
---
 backends/hostmem.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/backends/hostmem.c b/backends/hostmem.c
index f61093654e..5c71bd3f6b 100644
--- a/backends/hostmem.c
+++ b/backends/hostmem.c
@@ -18,6 +18,7 @@
 #include "qapi/visitor.h"
 #include "qemu/config-file.h"
 #include "qom/object_interfaces.h"
+#include "migration/misc.h"
 
 #ifdef CONFIG_NUMA
 #include <numaif.h>
@@ -271,6 +272,11 @@ host_memory_backend_memory_complete(UserCreatable *uc, 
Error **errp)
     void *ptr;
     uint64_t sz;
 
+    if (!migration_is_idle()) {
+        error_setg(errp, "Adding memory-backend isn't allowed while 
migrating");
+        goto out;
+    }
+
     if (bc->alloc) {
         bc->alloc(backend, &local_err);
         if (local_err) {
@@ -344,7 +350,8 @@ out:
 static bool
 host_memory_backend_can_be_deleted(UserCreatable *uc)
 {
-    if (host_memory_backend_is_mapped(MEMORY_BACKEND(uc))) {
+    if (host_memory_backend_is_mapped(MEMORY_BACKEND(uc)) ||
+        !migration_is_idle()) {
         return false;
     } else {
         return true;
-- 
2.21.0