[PATCH 0/2] migration/multifd: Fix rb->receivedmap cleanup race

[PATCH 0/2] migration/multifd: Fix rb->receivedmap cleanup race

[Fabiano Rosas]

This fixes the crash we've been seing recently in migration-test. The
first patch is a cleanup to have only one place calling
qemu_loadvm_state_cleanup() and the second patch reorders the cleanup
calls to make multifd_recv_cleanup() run first and stop the recv
threads.

CI run: https://gitlab.com/farosas/qemu/-/pipelines/1453038652

Fabiano Rosas (2):
  migration/savevm: Remove extra load cleanup calls
  migration/multifd: Fix rb->receivedmap cleanup race

 migration/migration.c |  1 +
 migration/migration.h |  1 -
 migration/savevm.c    | 11 -----------
 3 files changed, 1 insertion(+), 12 deletions(-)

-- 
2.35.3

[PATCH 1/2] migration/savevm: Remove extra load cleanup calls

[Fabiano Rosas]

There are two qemu_loadvm_state_cleanup() calls that were introduced
when qemu_loadvm_state_setup() was still called before loading the
configuration section, so there was state to be cleaned up if the
header checks failed.

However, commit 9e14b84908 ("migration/savevm: load_header before
load_setup") has moved that configuration section part to
qemu_loadvm_state_header() which now happens before
qemu_loadvm_state_setup().

Remove the cleanup calls that are now misplaced.

CC: qemu-stable@nongnu.org
Fixes: 9e14b84908 ("migration/savevm: load_header before load_setup")
Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 migration/savevm.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/migration/savevm.c b/migration/savevm.c
index d500eae979..d0759694fd 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -2732,13 +2732,11 @@ static int qemu_loadvm_state_header(QEMUFile *f)
     if (migrate_get_current()->send_configuration) {
         if (qemu_get_byte(f) != QEMU_VM_CONFIGURATION) {
             error_report("Configuration section missing");
-            qemu_loadvm_state_cleanup();
             return -EINVAL;
         }
         ret = vmstate_load_state(f, &vmstate_configuration, &savevm_state, 0);
 
         if (ret) {
-            qemu_loadvm_state_cleanup();
             return ret;
         }
     }
-- 
2.35.3

[PATCH 2/2] migration/multifd: Fix rb->receivedmap cleanup race

[Fabiano Rosas]

Fix a segmentation fault in multifd when rb->receivedmap is cleared
too early.

After commit 5ef7e26bdb ("migration/multifd: solve zero page causing
multiple page faults"), multifd started using the rb->receivedmap
bitmap, which belongs to ram.c and is initialized and *freed* from the
ram SaveVMHandlers.

Multifd threads are live until migration_incoming_state_destroy(),
which is called after qemu_loadvm_state_cleanup(), leading to a crash
when accessing rb->receivedmap.

process_incoming_migration_co()        ...
  qemu_loadvm_state()                  multifd_nocomp_recv()
    qemu_loadvm_state_cleanup()          ramblock_recv_bitmap_set_offset()
      rb->receivedmap = NULL               set_bit_atomic(..., rb->receivedmap)
  ...
  migration_incoming_state_destroy()
    multifd_recv_cleanup()
      multifd_recv_terminate_threads(NULL)

Move the loadvm cleanup into migration_incoming_state_destroy(), after
multifd_recv_cleanup() to ensure multifd thread have already exited
when rb->receivedmap is cleared.

The have_listen_thread logic can now be removed because its purpose
was to delay cleanup until postcopy_ram_listen_thread() had finished.

CC: qemu-stable@nongnu.org
Fixes: 5ef7e26bdb ("migration/multifd: solve zero page causing multiple page faults")
Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 migration/migration.c | 1 +
 migration/migration.h | 1 -
 migration/savevm.c    | 9 ---------
 3 files changed, 1 insertion(+), 10 deletions(-)

diff --git a/migration/migration.c b/migration/migration.c
index 3dea06d577..b190a574b1 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -378,6 +378,7 @@ void migration_incoming_state_destroy(void)
     struct MigrationIncomingState *mis = migration_incoming_get_current();
 
     multifd_recv_cleanup();
+    qemu_loadvm_state_cleanup();
 
     if (mis->to_src_file) {
         /* Tell source that we are done */
diff --git a/migration/migration.h b/migration/migration.h
index 38aa1402d5..20b0a5b66e 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -101,7 +101,6 @@ struct MigrationIncomingState {
     /* Set this when we want the fault thread to quit */
     bool           fault_thread_quit;
 
-    bool           have_listen_thread;
     QemuThread     listen_thread;
 
     /* For the kernel to send us notifications */
diff --git a/migration/savevm.c b/migration/savevm.c
index d0759694fd..532ee5e4b0 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -2076,10 +2076,8 @@ static void *postcopy_ram_listen_thread(void *opaque)
      * got a bad migration state).
      */
     migration_incoming_state_destroy();
-    qemu_loadvm_state_cleanup();
 
     rcu_unregister_thread();
-    mis->have_listen_thread = false;
     postcopy_state_set(POSTCOPY_INCOMING_END);
 
     object_unref(OBJECT(migr));
@@ -2130,7 +2128,6 @@ static int loadvm_postcopy_handle_listen(MigrationIncomingState *mis)
         return -1;
     }
 
-    mis->have_listen_thread = true;
     postcopy_thread_create(mis, &mis->listen_thread, "mig/dst/listen",
                            postcopy_ram_listen_thread, QEMU_THREAD_DETACHED);
     trace_loadvm_postcopy_handle_listen("return");
@@ -2978,11 +2975,6 @@ int qemu_loadvm_state(QEMUFile *f)
 
     trace_qemu_loadvm_state_post_main(ret);
 
-    if (mis->have_listen_thread) {
-        /* Listen thread still going, can't clean up yet */
-        return ret;
-    }
-
     if (ret == 0) {
         ret = qemu_file_get_error(f);
     }
@@ -3022,7 +3014,6 @@ int qemu_loadvm_state(QEMUFile *f)
         }
     }
 
-    qemu_loadvm_state_cleanup();
     cpu_synchronize_all_post_init();
 
     return ret;
-- 
2.35.3

Re: [PATCH 1/2] migration/savevm: Remove extra load cleanup calls

[Peter Xu]

On Fri, Sep 13, 2024 at 07:05:41PM -0300, Fabiano Rosas wrote:
> There are two qemu_loadvm_state_cleanup() calls that were introduced
> when qemu_loadvm_state_setup() was still called before loading the
> configuration section, so there was state to be cleaned up if the
> header checks failed.
> 
> However, commit 9e14b84908 ("migration/savevm: load_header before
> load_setup") has moved that configuration section part to
> qemu_loadvm_state_header() which now happens before
> qemu_loadvm_state_setup().
> 
> Remove the cleanup calls that are now misplaced.
> 
> CC: qemu-stable@nongnu.org
> Fixes: 9e14b84908 ("migration/savevm: load_header before load_setup")
> Signed-off-by: Fabiano Rosas <farosas@suse.de>

Reviewed-by: Peter Xu <peterx@redhat.com>

We don't need to copy stable, am I right?  IIUC it's a good cleanup,
however not a bug fix, as qemu_loadvm_state_cleanup() can be invoked
without calling _setup() safely?

> ---
>  migration/savevm.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/migration/savevm.c b/migration/savevm.c
> index d500eae979..d0759694fd 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -2732,13 +2732,11 @@ static int qemu_loadvm_state_header(QEMUFile *f)
>      if (migrate_get_current()->send_configuration) {
>          if (qemu_get_byte(f) != QEMU_VM_CONFIGURATION) {
>              error_report("Configuration section missing");
> -            qemu_loadvm_state_cleanup();
>              return -EINVAL;
>          }
>          ret = vmstate_load_state(f, &vmstate_configuration, &savevm_state, 0);
>  
>          if (ret) {
> -            qemu_loadvm_state_cleanup();
>              return ret;
>          }
>      }
> -- 
> 2.35.3
> 

-- 
Peter Xu

Re: [PATCH 2/2] migration/multifd: Fix rb->receivedmap cleanup race

[Peter Xu]

On Fri, Sep 13, 2024 at 07:05:42PM -0300, Fabiano Rosas wrote:
> Fix a segmentation fault in multifd when rb->receivedmap is cleared
> too early.
> 
> After commit 5ef7e26bdb ("migration/multifd: solve zero page causing
> multiple page faults"), multifd started using the rb->receivedmap
> bitmap, which belongs to ram.c and is initialized and *freed* from the
> ram SaveVMHandlers.
> 
> Multifd threads are live until migration_incoming_state_destroy(),
> which is called after qemu_loadvm_state_cleanup(), leading to a crash
> when accessing rb->receivedmap.
> 
> process_incoming_migration_co()        ...
>   qemu_loadvm_state()                  multifd_nocomp_recv()
>     qemu_loadvm_state_cleanup()          ramblock_recv_bitmap_set_offset()
>       rb->receivedmap = NULL               set_bit_atomic(..., rb->receivedmap)
>   ...
>   migration_incoming_state_destroy()
>     multifd_recv_cleanup()
>       multifd_recv_terminate_threads(NULL)
> 
> Move the loadvm cleanup into migration_incoming_state_destroy(), after
> multifd_recv_cleanup() to ensure multifd thread have already exited
> when rb->receivedmap is cleared.
> 
> The have_listen_thread logic can now be removed because its purpose
> was to delay cleanup until postcopy_ram_listen_thread() had finished.
> 
> CC: qemu-stable@nongnu.org
> Fixes: 5ef7e26bdb ("migration/multifd: solve zero page causing multiple page faults")
> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> ---
>  migration/migration.c | 1 +
>  migration/migration.h | 1 -
>  migration/savevm.c    | 9 ---------
>  3 files changed, 1 insertion(+), 10 deletions(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index 3dea06d577..b190a574b1 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -378,6 +378,7 @@ void migration_incoming_state_destroy(void)
>      struct MigrationIncomingState *mis = migration_incoming_get_current();
>  
>      multifd_recv_cleanup();
> +    qemu_loadvm_state_cleanup();
>  
>      if (mis->to_src_file) {
>          /* Tell source that we are done */
> diff --git a/migration/migration.h b/migration/migration.h
> index 38aa1402d5..20b0a5b66e 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -101,7 +101,6 @@ struct MigrationIncomingState {
>      /* Set this when we want the fault thread to quit */
>      bool           fault_thread_quit;
>  
> -    bool           have_listen_thread;
>      QemuThread     listen_thread;
>  
>      /* For the kernel to send us notifications */
> diff --git a/migration/savevm.c b/migration/savevm.c
> index d0759694fd..532ee5e4b0 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -2076,10 +2076,8 @@ static void *postcopy_ram_listen_thread(void *opaque)
>       * got a bad migration state).
>       */
>      migration_incoming_state_destroy();
> -    qemu_loadvm_state_cleanup();
>  
>      rcu_unregister_thread();
> -    mis->have_listen_thread = false;
>      postcopy_state_set(POSTCOPY_INCOMING_END);
>  
>      object_unref(OBJECT(migr));
> @@ -2130,7 +2128,6 @@ static int loadvm_postcopy_handle_listen(MigrationIncomingState *mis)
>          return -1;
>      }
>  
> -    mis->have_listen_thread = true;
>      postcopy_thread_create(mis, &mis->listen_thread, "mig/dst/listen",
>                             postcopy_ram_listen_thread, QEMU_THREAD_DETACHED);
>      trace_loadvm_postcopy_handle_listen("return");
> @@ -2978,11 +2975,6 @@ int qemu_loadvm_state(QEMUFile *f)
>  
>      trace_qemu_loadvm_state_post_main(ret);
>  
> -    if (mis->have_listen_thread) {
> -        /* Listen thread still going, can't clean up yet */
> -        return ret;
> -    }

Hmm, I wonder whether we would still need this.  IIUC it's not only about
cleanup, but also that when postcopy is involved, dst QEMU postpones doing
any of the rest in the qemu_loadvm_state_main() call.

E.g. cpu put, aka, cpu_synchronize_all_post_init(), is also done in
loadvm_postcopy_handle_run_bh() later.

IOW, I'd then expect when this patch applied we'll put cpu twice?

I think the should_send_vmdesc() part is fine, as it returns false for
postcopy anyway.  However not sure on the cpu post_init above.

> -
>      if (ret == 0) {
>          ret = qemu_file_get_error(f);
>      }
> @@ -3022,7 +3014,6 @@ int qemu_loadvm_state(QEMUFile *f)
>          }
>      }
>  
> -    qemu_loadvm_state_cleanup();
>      cpu_synchronize_all_post_init();
>  
>      return ret;
> -- 
> 2.35.3
> 

-- 
Peter Xu

Re: [PATCH 1/2] migration/savevm: Remove extra load cleanup calls

[Fabiano Rosas]

Peter Xu <peterx@redhat.com> writes:

> On Fri, Sep 13, 2024 at 07:05:41PM -0300, Fabiano Rosas wrote:
>> There are two qemu_loadvm_state_cleanup() calls that were introduced
>> when qemu_loadvm_state_setup() was still called before loading the
>> configuration section, so there was state to be cleaned up if the
>> header checks failed.
>> 
>> However, commit 9e14b84908 ("migration/savevm: load_header before
>> load_setup") has moved that configuration section part to
>> qemu_loadvm_state_header() which now happens before
>> qemu_loadvm_state_setup().
>> 
>> Remove the cleanup calls that are now misplaced.
>> 
>> CC: qemu-stable@nongnu.org
>> Fixes: 9e14b84908 ("migration/savevm: load_header before load_setup")
>> Signed-off-by: Fabiano Rosas <farosas@suse.de>
>
> Reviewed-by: Peter Xu <peterx@redhat.com>
>
> We don't need to copy stable, am I right?  IIUC it's a good cleanup,
> however not a bug fix, as qemu_loadvm_state_cleanup() can be invoked
> without calling _setup() safely?

Hm, I think you're right. If we fail in the header part the multifd
threads will still be waiting for the ram code to release them.

Re: [PATCH 2/2] migration/multifd: Fix rb->receivedmap cleanup race

[Fabiano Rosas]

Peter Xu <peterx@redhat.com> writes:

> On Fri, Sep 13, 2024 at 07:05:42PM -0300, Fabiano Rosas wrote:
>> Fix a segmentation fault in multifd when rb->receivedmap is cleared
>> too early.
>> 
>> After commit 5ef7e26bdb ("migration/multifd: solve zero page causing
>> multiple page faults"), multifd started using the rb->receivedmap
>> bitmap, which belongs to ram.c and is initialized and *freed* from the
>> ram SaveVMHandlers.
>> 
>> Multifd threads are live until migration_incoming_state_destroy(),
>> which is called after qemu_loadvm_state_cleanup(), leading to a crash
>> when accessing rb->receivedmap.
>> 
>> process_incoming_migration_co()        ...
>>   qemu_loadvm_state()                  multifd_nocomp_recv()
>>     qemu_loadvm_state_cleanup()          ramblock_recv_bitmap_set_offset()
>>       rb->receivedmap = NULL               set_bit_atomic(..., rb->receivedmap)
>>   ...
>>   migration_incoming_state_destroy()
>>     multifd_recv_cleanup()
>>       multifd_recv_terminate_threads(NULL)
>> 
>> Move the loadvm cleanup into migration_incoming_state_destroy(), after
>> multifd_recv_cleanup() to ensure multifd thread have already exited
>> when rb->receivedmap is cleared.
>> 
>> The have_listen_thread logic can now be removed because its purpose
>> was to delay cleanup until postcopy_ram_listen_thread() had finished.
>> 
>> CC: qemu-stable@nongnu.org
>> Fixes: 5ef7e26bdb ("migration/multifd: solve zero page causing multiple page faults")
>> Signed-off-by: Fabiano Rosas <farosas@suse.de>
>> ---
>>  migration/migration.c | 1 +
>>  migration/migration.h | 1 -
>>  migration/savevm.c    | 9 ---------
>>  3 files changed, 1 insertion(+), 10 deletions(-)
>> 
>> diff --git a/migration/migration.c b/migration/migration.c
>> index 3dea06d577..b190a574b1 100644
>> --- a/migration/migration.c
>> +++ b/migration/migration.c
>> @@ -378,6 +378,7 @@ void migration_incoming_state_destroy(void)
>>      struct MigrationIncomingState *mis = migration_incoming_get_current();
>>  
>>      multifd_recv_cleanup();
>> +    qemu_loadvm_state_cleanup();
>>  
>>      if (mis->to_src_file) {
>>          /* Tell source that we are done */
>> diff --git a/migration/migration.h b/migration/migration.h
>> index 38aa1402d5..20b0a5b66e 100644
>> --- a/migration/migration.h
>> +++ b/migration/migration.h
>> @@ -101,7 +101,6 @@ struct MigrationIncomingState {
>>      /* Set this when we want the fault thread to quit */
>>      bool           fault_thread_quit;
>>  
>> -    bool           have_listen_thread;
>>      QemuThread     listen_thread;
>>  
>>      /* For the kernel to send us notifications */
>> diff --git a/migration/savevm.c b/migration/savevm.c
>> index d0759694fd..532ee5e4b0 100644
>> --- a/migration/savevm.c
>> +++ b/migration/savevm.c
>> @@ -2076,10 +2076,8 @@ static void *postcopy_ram_listen_thread(void *opaque)
>>       * got a bad migration state).
>>       */
>>      migration_incoming_state_destroy();
>> -    qemu_loadvm_state_cleanup();
>>  
>>      rcu_unregister_thread();
>> -    mis->have_listen_thread = false;
>>      postcopy_state_set(POSTCOPY_INCOMING_END);
>>  
>>      object_unref(OBJECT(migr));
>> @@ -2130,7 +2128,6 @@ static int loadvm_postcopy_handle_listen(MigrationIncomingState *mis)
>>          return -1;
>>      }
>>  
>> -    mis->have_listen_thread = true;
>>      postcopy_thread_create(mis, &mis->listen_thread, "mig/dst/listen",
>>                             postcopy_ram_listen_thread, QEMU_THREAD_DETACHED);
>>      trace_loadvm_postcopy_handle_listen("return");
>> @@ -2978,11 +2975,6 @@ int qemu_loadvm_state(QEMUFile *f)
>>  
>>      trace_qemu_loadvm_state_post_main(ret);
>>  
>> -    if (mis->have_listen_thread) {
>> -        /* Listen thread still going, can't clean up yet */
>> -        return ret;
>> -    }
>
> Hmm, I wonder whether we would still need this.  IIUC it's not only about
> cleanup, but also that when postcopy is involved, dst QEMU postpones doing
> any of the rest in the qemu_loadvm_state_main() call.
>
> E.g. cpu put, aka, cpu_synchronize_all_post_init(), is also done in
> loadvm_postcopy_handle_run_bh() later.
>
> IOW, I'd then expect when this patch applied we'll put cpu twice?
>
> I think the should_send_vmdesc() part is fine, as it returns false for
> postcopy anyway.  However not sure on the cpu post_init above.

I'm not sure either, but there's several ioctls in there, so it's
probably better to skip them. I'll keep the have_listen and adjust the
comment.

[PATCH 2/2] migration/multifd: Fix rb->receivedmap cleanup race

[Fabiano Rosas]

Fix a segmentation fault in multifd when rb->receivedmap is cleared
too early.

After commit 5ef7e26bdb ("migration/multifd: solve zero page causing
multiple page faults"), multifd started using the rb->receivedmap
bitmap, which belongs to ram.c and is initialized and *freed* from the
ram SaveVMHandlers.

Multifd threads are live until migration_incoming_state_destroy(),
which is called after qemu_loadvm_state_cleanup(), leading to a crash
when accessing rb->receivedmap.

process_incoming_migration_co()        ...
  qemu_loadvm_state()                  multifd_nocomp_recv()
    qemu_loadvm_state_cleanup()          ramblock_recv_bitmap_set_offset()
      rb->receivedmap = NULL               set_bit_atomic(..., rb->receivedmap)
  ...
  migration_incoming_state_destroy()
    multifd_recv_cleanup()
      multifd_recv_terminate_threads(NULL)

Move the loadvm cleanup into migration_incoming_state_destroy(), after
multifd_recv_cleanup() to ensure multifd threads have already exited
when rb->receivedmap is cleared.

Adjust the postcopy listen thread comment to indicate that we still
want to skip the cpu synchronization.

CC: qemu-stable@nongnu.org
Fixes: 5ef7e26bdb ("migration/multifd: solve zero page causing multiple page faults")
Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 migration/migration.c | 1 +
 migration/savevm.c    | 6 ++++--
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/migration/migration.c b/migration/migration.c
index 3dea06d577..b190a574b1 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -378,6 +378,7 @@ void migration_incoming_state_destroy(void)
     struct MigrationIncomingState *mis = migration_incoming_get_current();
 
     multifd_recv_cleanup();
+    qemu_loadvm_state_cleanup();
 
     if (mis->to_src_file) {
         /* Tell source that we are done */
diff --git a/migration/savevm.c b/migration/savevm.c
index d0759694fd..7e1e27182a 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -2979,7 +2979,10 @@ int qemu_loadvm_state(QEMUFile *f)
     trace_qemu_loadvm_state_post_main(ret);
 
     if (mis->have_listen_thread) {
-        /* Listen thread still going, can't clean up yet */
+        /*
+         * Postcopy listen thread still going, don't synchronize the
+         * cpus yet.
+         */
         return ret;
     }
 
@@ -3022,7 +3025,6 @@ int qemu_loadvm_state(QEMUFile *f)
         }
     }
 
-    qemu_loadvm_state_cleanup();
     cpu_synchronize_all_post_init();
 
     return ret;
-- 
2.35.3

Re: [PATCH 2/2] migration/multifd: Fix rb->receivedmap cleanup race

[Peter Xu]

On Tue, Sep 17, 2024 at 03:58:02PM -0300, Fabiano Rosas wrote:
> Fix a segmentation fault in multifd when rb->receivedmap is cleared
> too early.
> 
> After commit 5ef7e26bdb ("migration/multifd: solve zero page causing
> multiple page faults"), multifd started using the rb->receivedmap
> bitmap, which belongs to ram.c and is initialized and *freed* from the
> ram SaveVMHandlers.
> 
> Multifd threads are live until migration_incoming_state_destroy(),
> which is called after qemu_loadvm_state_cleanup(), leading to a crash
> when accessing rb->receivedmap.
> 
> process_incoming_migration_co()        ...
>   qemu_loadvm_state()                  multifd_nocomp_recv()
>     qemu_loadvm_state_cleanup()          ramblock_recv_bitmap_set_offset()
>       rb->receivedmap = NULL               set_bit_atomic(..., rb->receivedmap)
>   ...
>   migration_incoming_state_destroy()
>     multifd_recv_cleanup()
>       multifd_recv_terminate_threads(NULL)
> 
> Move the loadvm cleanup into migration_incoming_state_destroy(), after
> multifd_recv_cleanup() to ensure multifd threads have already exited
> when rb->receivedmap is cleared.
> 
> Adjust the postcopy listen thread comment to indicate that we still
> want to skip the cpu synchronization.
> 
> CC: qemu-stable@nongnu.org
> Fixes: 5ef7e26bdb ("migration/multifd: solve zero page causing multiple page faults")
> Signed-off-by: Fabiano Rosas <farosas@suse.de>

Reviewed-by: Peter Xu <peterx@redhat.com>

One trivial question below..

> ---
>  migration/migration.c | 1 +
>  migration/savevm.c    | 6 ++++--
>  2 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index 3dea06d577..b190a574b1 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -378,6 +378,7 @@ void migration_incoming_state_destroy(void)
>      struct MigrationIncomingState *mis = migration_incoming_get_current();
>  
>      multifd_recv_cleanup();

Would you mind I add a comment squashed here when queue?

       /*
        * RAM state cleanup needs to happen after multifd cleanup, because
        * multifd threads can use some of its states (receivedmap).
        */

> +    qemu_loadvm_state_cleanup();
>  
>      if (mis->to_src_file) {
>          /* Tell source that we are done */
> diff --git a/migration/savevm.c b/migration/savevm.c
> index d0759694fd..7e1e27182a 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -2979,7 +2979,10 @@ int qemu_loadvm_state(QEMUFile *f)
>      trace_qemu_loadvm_state_post_main(ret);
>  
>      if (mis->have_listen_thread) {
> -        /* Listen thread still going, can't clean up yet */
> +        /*
> +         * Postcopy listen thread still going, don't synchronize the
> +         * cpus yet.
> +         */
>          return ret;
>      }
>  
> @@ -3022,7 +3025,6 @@ int qemu_loadvm_state(QEMUFile *f)
>          }
>      }
>  
> -    qemu_loadvm_state_cleanup();
>      cpu_synchronize_all_post_init();
>  
>      return ret;
> -- 
> 2.35.3
> 

-- 
Peter Xu

Re: [PATCH 2/2] migration/multifd: Fix rb->receivedmap cleanup race

[Fabiano Rosas]

Peter Xu <peterx@redhat.com> writes:

> On Tue, Sep 17, 2024 at 03:58:02PM -0300, Fabiano Rosas wrote:
>> Fix a segmentation fault in multifd when rb->receivedmap is cleared
>> too early.
>> 
>> After commit 5ef7e26bdb ("migration/multifd: solve zero page causing
>> multiple page faults"), multifd started using the rb->receivedmap
>> bitmap, which belongs to ram.c and is initialized and *freed* from the
>> ram SaveVMHandlers.
>> 
>> Multifd threads are live until migration_incoming_state_destroy(),
>> which is called after qemu_loadvm_state_cleanup(), leading to a crash
>> when accessing rb->receivedmap.
>> 
>> process_incoming_migration_co()        ...
>>   qemu_loadvm_state()                  multifd_nocomp_recv()
>>     qemu_loadvm_state_cleanup()          ramblock_recv_bitmap_set_offset()
>>       rb->receivedmap = NULL               set_bit_atomic(..., rb->receivedmap)
>>   ...
>>   migration_incoming_state_destroy()
>>     multifd_recv_cleanup()
>>       multifd_recv_terminate_threads(NULL)
>> 
>> Move the loadvm cleanup into migration_incoming_state_destroy(), after
>> multifd_recv_cleanup() to ensure multifd threads have already exited
>> when rb->receivedmap is cleared.
>> 
>> Adjust the postcopy listen thread comment to indicate that we still
>> want to skip the cpu synchronization.
>> 
>> CC: qemu-stable@nongnu.org
>> Fixes: 5ef7e26bdb ("migration/multifd: solve zero page causing multiple page faults")
>> Signed-off-by: Fabiano Rosas <farosas@suse.de>
>
> Reviewed-by: Peter Xu <peterx@redhat.com>
>
> One trivial question below..
>
>> ---
>>  migration/migration.c | 1 +
>>  migration/savevm.c    | 6 ++++--
>>  2 files changed, 5 insertions(+), 2 deletions(-)
>> 
>> diff --git a/migration/migration.c b/migration/migration.c
>> index 3dea06d577..b190a574b1 100644
>> --- a/migration/migration.c
>> +++ b/migration/migration.c
>> @@ -378,6 +378,7 @@ void migration_incoming_state_destroy(void)
>>      struct MigrationIncomingState *mis = migration_incoming_get_current();
>>  
>>      multifd_recv_cleanup();
>
> Would you mind I add a comment squashed here when queue?
>
>        /*
>         * RAM state cleanup needs to happen after multifd cleanup, because
>         * multifd threads can use some of its states (receivedmap).
>         */

Yeah, that's ok.

>
>> +    qemu_loadvm_state_cleanup();
>>  
>>      if (mis->to_src_file) {
>>          /* Tell source that we are done */
>> diff --git a/migration/savevm.c b/migration/savevm.c
>> index d0759694fd..7e1e27182a 100644
>> --- a/migration/savevm.c
>> +++ b/migration/savevm.c
>> @@ -2979,7 +2979,10 @@ int qemu_loadvm_state(QEMUFile *f)
>>      trace_qemu_loadvm_state_post_main(ret);
>>  
>>      if (mis->have_listen_thread) {
>> -        /* Listen thread still going, can't clean up yet */
>> +        /*
>> +         * Postcopy listen thread still going, don't synchronize the
>> +         * cpus yet.
>> +         */
>>          return ret;
>>      }
>>  
>> @@ -3022,7 +3025,6 @@ int qemu_loadvm_state(QEMUFile *f)
>>          }
>>      }
>>  
>> -    qemu_loadvm_state_cleanup();
>>      cpu_synchronize_all_post_init();
>>  
>>      return ret;
>> -- 
>> 2.35.3
>> 

Re: [PATCH 2/2] migration/multifd: Fix rb->receivedmap cleanup race

[Elena Ufimtseva]

[-- Attachment #1: Type: text/plain, Size: 4651 bytes --]

On Fri, Sep 13, 2024 at 3:07 PM Fabiano Rosas <farosas@suse.de> wrote:

> Fix a segmentation fault in multifd when rb->receivedmap is cleared
> too early.
>
> After commit 5ef7e26bdb ("migration/multifd: solve zero page causing
> multiple page faults"), multifd started using the rb->receivedmap
> bitmap, which belongs to ram.c and is initialized and *freed* from the
> ram SaveVMHandlers.
>
> Multifd threads are live until migration_incoming_state_destroy(),
> which is called after qemu_loadvm_state_cleanup(), leading to a crash
> when accessing rb->receivedmap.
>
> process_incoming_migration_co()        ...
>   qemu_loadvm_state()                  multifd_nocomp_recv()
>     qemu_loadvm_state_cleanup()          ramblock_recv_bitmap_set_offset()
>       rb->receivedmap = NULL               set_bit_atomic(...,
> rb->receivedmap)
>   ...
>   migration_incoming_state_destroy()
>     multifd_recv_cleanup()
>       multifd_recv_terminate_threads(NULL)
>
> Move the loadvm cleanup into migration_incoming_state_destroy(), after
> multifd_recv_cleanup() to ensure multifd thread have already exited
> when rb->receivedmap is cleared.
>
> The have_listen_thread logic can now be removed because its purpose
> was to delay cleanup until postcopy_ram_listen_thread() had finished.
>
> CC: qemu-stable@nongnu.org
> Fixes: 5ef7e26bdb ("migration/multifd: solve zero page causing multiple
> page faults")
> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> ---
>  migration/migration.c | 1 +
>  migration/migration.h | 1 -
>  migration/savevm.c    | 9 ---------
>  3 files changed, 1 insertion(+), 10 deletions(-)
>
> diff --git a/migration/migration.c b/migration/migration.c
> index 3dea06d577..b190a574b1 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -378,6 +378,7 @@ void migration_incoming_state_destroy(void)
>      struct MigrationIncomingState *mis = migration_incoming_get_current();
>
>      multifd_recv_cleanup();
> +    qemu_loadvm_state_cleanup();
>
>      if (mis->to_src_file) {
>          /* Tell source that we are done */
> diff --git a/migration/migration.h b/migration/migration.h
> index 38aa1402d5..20b0a5b66e 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -101,7 +101,6 @@ struct MigrationIncomingState {
>      /* Set this when we want the fault thread to quit */
>      bool           fault_thread_quit;
>
> -    bool           have_listen_thread;
>      QemuThread     listen_thread;
>
>      /* For the kernel to send us notifications */
> diff --git a/migration/savevm.c b/migration/savevm.c
> index d0759694fd..532ee5e4b0 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -2076,10 +2076,8 @@ static void *postcopy_ram_listen_thread(void
> *opaque)
>       * got a bad migration state).
>       */
>      migration_incoming_state_destroy();
> -    qemu_loadvm_state_cleanup();
>
>      rcu_unregister_thread();
> -    mis->have_listen_thread = false;
>      postcopy_state_set(POSTCOPY_INCOMING_END);
>
>      object_unref(OBJECT(migr));
> @@ -2130,7 +2128,6 @@ static int
> loadvm_postcopy_handle_listen(MigrationIncomingState *mis)
>          return -1;
>      }
>
> -    mis->have_listen_thread = true;
>      postcopy_thread_create(mis, &mis->listen_thread, "mig/dst/listen",
>                             postcopy_ram_listen_thread,
> QEMU_THREAD_DETACHED);
>      trace_loadvm_postcopy_handle_listen("return");
> @@ -2978,11 +2975,6 @@ int qemu_loadvm_state(QEMUFile *f)
>
>      trace_qemu_loadvm_state_post_main(ret);
>
> -    if (mis->have_listen_thread) {
> -        /* Listen thread still going, can't clean up yet */
> -        return ret;
> -    }
> -
>      if (ret == 0) {
>          ret = qemu_file_get_error(f);
>      }
> @@ -3022,7 +3014,6 @@ int qemu_loadvm_state(QEMUFile *f)
>          }
>      }
>
> -    qemu_loadvm_state_cleanup();
>      cpu_synchronize_all_post_init();
>


Hi Fabiano

I have a question. By removing  qemu_loadvm_state_cleanup() here, the
failure path that ends up with exit(EXIT_FAILURE)
in process_incoming_migration_co() end up not calling the
qemu_loadvm_state_cleanup(). I am not sure how this is important since
there is exit, but the
vfio, for example, will not call the VF reset.

Another more general question is why destination Qemu has to terminate
there if there was an error detected during live migration?
Could just failing the migration and leave destination running be a more
expected scenario?

Thank you!

     return ret;
> --
> 2.35.3
>
>
>

-- 
Elena

[-- Attachment #2: Type: text/html, Size: 5926 bytes --]

Re: [PATCH 2/2] migration/multifd: Fix rb->receivedmap cleanup race

[Fabiano Rosas]

Elena Ufimtseva <ufimtseva@gmail.com> writes:

> On Fri, Sep 13, 2024 at 3:07 PM Fabiano Rosas <farosas@suse.de> wrote:
>
>> Fix a segmentation fault in multifd when rb->receivedmap is cleared
>> too early.
>>
>> After commit 5ef7e26bdb ("migration/multifd: solve zero page causing
>> multiple page faults"), multifd started using the rb->receivedmap
>> bitmap, which belongs to ram.c and is initialized and *freed* from the
>> ram SaveVMHandlers.
>>
>> Multifd threads are live until migration_incoming_state_destroy(),
>> which is called after qemu_loadvm_state_cleanup(), leading to a crash
>> when accessing rb->receivedmap.
>>
>> process_incoming_migration_co()        ...
>>   qemu_loadvm_state()                  multifd_nocomp_recv()
>>     qemu_loadvm_state_cleanup()          ramblock_recv_bitmap_set_offset()
>>       rb->receivedmap = NULL               set_bit_atomic(...,
>> rb->receivedmap)
>>   ...
>>   migration_incoming_state_destroy()
>>     multifd_recv_cleanup()
>>       multifd_recv_terminate_threads(NULL)
>>
>> Move the loadvm cleanup into migration_incoming_state_destroy(), after
>> multifd_recv_cleanup() to ensure multifd thread have already exited
>> when rb->receivedmap is cleared.
>>
>> The have_listen_thread logic can now be removed because its purpose
>> was to delay cleanup until postcopy_ram_listen_thread() had finished.
>>
>> CC: qemu-stable@nongnu.org
>> Fixes: 5ef7e26bdb ("migration/multifd: solve zero page causing multiple
>> page faults")
>> Signed-off-by: Fabiano Rosas <farosas@suse.de>
>> ---
>>  migration/migration.c | 1 +
>>  migration/migration.h | 1 -
>>  migration/savevm.c    | 9 ---------
>>  3 files changed, 1 insertion(+), 10 deletions(-)
>>
>> diff --git a/migration/migration.c b/migration/migration.c
>> index 3dea06d577..b190a574b1 100644
>> --- a/migration/migration.c
>> +++ b/migration/migration.c
>> @@ -378,6 +378,7 @@ void migration_incoming_state_destroy(void)
>>      struct MigrationIncomingState *mis = migration_incoming_get_current();
>>
>>      multifd_recv_cleanup();
>> +    qemu_loadvm_state_cleanup();
>>
>>      if (mis->to_src_file) {
>>          /* Tell source that we are done */
>> diff --git a/migration/migration.h b/migration/migration.h
>> index 38aa1402d5..20b0a5b66e 100644
>> --- a/migration/migration.h
>> +++ b/migration/migration.h
>> @@ -101,7 +101,6 @@ struct MigrationIncomingState {
>>      /* Set this when we want the fault thread to quit */
>>      bool           fault_thread_quit;
>>
>> -    bool           have_listen_thread;
>>      QemuThread     listen_thread;
>>
>>      /* For the kernel to send us notifications */
>> diff --git a/migration/savevm.c b/migration/savevm.c
>> index d0759694fd..532ee5e4b0 100644
>> --- a/migration/savevm.c
>> +++ b/migration/savevm.c
>> @@ -2076,10 +2076,8 @@ static void *postcopy_ram_listen_thread(void
>> *opaque)
>>       * got a bad migration state).
>>       */
>>      migration_incoming_state_destroy();
>> -    qemu_loadvm_state_cleanup();
>>
>>      rcu_unregister_thread();
>> -    mis->have_listen_thread = false;
>>      postcopy_state_set(POSTCOPY_INCOMING_END);
>>
>>      object_unref(OBJECT(migr));
>> @@ -2130,7 +2128,6 @@ static int
>> loadvm_postcopy_handle_listen(MigrationIncomingState *mis)
>>          return -1;
>>      }
>>
>> -    mis->have_listen_thread = true;
>>      postcopy_thread_create(mis, &mis->listen_thread, "mig/dst/listen",
>>                             postcopy_ram_listen_thread,
>> QEMU_THREAD_DETACHED);
>>      trace_loadvm_postcopy_handle_listen("return");
>> @@ -2978,11 +2975,6 @@ int qemu_loadvm_state(QEMUFile *f)
>>
>>      trace_qemu_loadvm_state_post_main(ret);
>>
>> -    if (mis->have_listen_thread) {
>> -        /* Listen thread still going, can't clean up yet */
>> -        return ret;
>> -    }
>> -
>>      if (ret == 0) {
>>          ret = qemu_file_get_error(f);
>>      }
>> @@ -3022,7 +3014,6 @@ int qemu_loadvm_state(QEMUFile *f)
>>          }
>>      }
>>
>> -    qemu_loadvm_state_cleanup();
>>      cpu_synchronize_all_post_init();
>>
>
>
> Hi Fabiano

Hi, sorry for the delay.

>
> I have a question. By removing  qemu_loadvm_state_cleanup() here, the
> failure path that ends up with exit(EXIT_FAILURE)
> in process_incoming_migration_co() end up not calling the
> qemu_loadvm_state_cleanup(). I am not sure how this is important since
> there is exit, but the
> vfio, for example, will not call the VF reset.

In the fail label, migration_incoming_state_destroy() is called right
before the exit block.

>
> Another more general question is why destination Qemu has to terminate
> there if there was an error detected during live migration?
> Could just failing the migration and leave destination running be a more
> expected scenario?

After failure to load, the destination VM is not really usable, so
terminating it seems ok. This was changed with the addition of
mis->exit_on_error in dbea1c89da ("qapi: introduce exit-on-error
parameter for migrate-incoming"). We still exit on error by default, but
allow the user to modify the behavior to allow management applications
to collect the error or inspect the VM.

>
> Thank you!
>
>      return ret;
>> --
>> 2.35.3
>>
>>
>>