Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating

[David Hildenbrand <david@redhat.com>]

On 14.02.20 18:29, Peter Xu wrote:
> On Fri, Feb 14, 2020 at 01:02:46PM +0100, David Hildenbrand wrote:
>> From c0049ac2e95d6756037db918852c507fb88297d9 Mon Sep 17 00:00:00 2001
>> From: David Hildenbrand <david@redhat.com>
>> Date: Fri, 14 Feb 2020 13:01:03 +0100
>> Subject: [PATCH v1] tmp
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>  migration/migration.c |  9 +++++++--
>>  migration/migration.h |  1 +
>>  migration/ram.c       | 42 ++++++++++++++++++++++++++++++++++++++++++
>>  3 files changed, 50 insertions(+), 2 deletions(-)
>>
>> diff --git a/migration/migration.c b/migration/migration.c
>> index 3a21a4686c..0e7efe2920 100644
>> --- a/migration/migration.c
>> +++ b/migration/migration.c
>> @@ -175,13 +175,18 @@ void migration_object_init(void)
>>      }
>>  }
>>  
>> +void migration_cancel(void)
>> +{
>> +    migrate_fd_cancel(current_migration);
>> +}
>> +
>>  void migration_shutdown(void)
>>  {
>>      /*
>>       * Cancel the current migration - that will (eventually)
>>       * stop the migration using this structure
>>       */
>> -    migrate_fd_cancel(current_migration);
>> +    migration_cancel();
>>      object_unref(OBJECT(current_migration));
>>  }
>>  
>> @@ -2019,7 +2024,7 @@ void qmp_migrate(const char *uri, bool has_blk, bool blk,
>>  
>>  void qmp_migrate_cancel(Error **errp)
>>  {
>> -    migrate_fd_cancel(migrate_get_current());
>> +    migration_cancel();
>>  }
>>  
>>  void qmp_migrate_continue(MigrationStatus state, Error **errp)
>> diff --git a/migration/migration.h b/migration/migration.h
>> index 8473ddfc88..79fd74afa5 100644
>> --- a/migration/migration.h
>> +++ b/migration/migration.h
>> @@ -343,5 +343,6 @@ int foreach_not_ignored_block(RAMBlockIterFunc func, void *opaque);
>>  void migration_make_urgent_request(void);
>>  void migration_consume_urgent_request(void);
>>  bool migration_rate_limit(void);
>> +void migration_cancel(void);
>>  
>>  #endif
>> diff --git a/migration/ram.c b/migration/ram.c
>> index ed23ed1c7c..f86f32b453 100644
>> --- a/migration/ram.c
>> +++ b/migration/ram.c
>> @@ -52,6 +52,7 @@
>>  #include "migration/colo.h"
>>  #include "block.h"
>>  #include "sysemu/sysemu.h"
>> +#include "sysemu/runstate.h"
>>  #include "savevm.h"
>>  #include "qemu/iov.h"
>>  #include "multifd.h"
>> @@ -3710,8 +3711,49 @@ static SaveVMHandlers savevm_ram_handlers = {
>>      .resume_prepare = ram_resume_prepare,
>>  };
>>  
>> +static void ram_mig_ram_block_resized(RAMBlockNotifier *n, void *host,
>> +                                      size_t old_size, size_t new_size)
>> +{
>> +    /*
>> +     * We don't care about resizes triggered on incoming migration (when
>> +     * syncing ram blocks), or of course, when no migration is going on.
>> +     */
>> +    if (migration_is_idle() || !runstate_is_running()) {
>> +        return;
>> +    }
> 
> I feel like migration_is_idle() check is enough.  Firstly, I feel like
> we allow migration even with VM stopped.  At the meantime, if VM is
> not running, I see no reason that this resizing will happen after all? :)

Migration code resizes ram blocks when synchronizing the ram state. See
qemu_ram_resize() in ram_load_precopy()

That happens on incoming migration while the vm is stopped.

> 
>> +
>> +    if (!postcopy_is_running()) {
>> +        Error *err = NULL;
>> +
>> +        /*
>> +         * Precopy code cannot deal with the size of ram blocks changing at
>> +         * random points in time. We're still running on the source, abort
>> +         * the migration and continue running here. Make sure to wait until
>> +         * migration was canceled.
>> +         */
>> +        error_setg(&err, "RAM resized during precopy.");
>> +        migrate_set_error(migrate_get_current(), err);
>> +        error_free(err);
>> +        migration_cancel();
>> +    } else {
>> +        /*
>> +         * Postcopy code cannot deal with the size of ram blocks changing at
>> +         * random points in time. We're running on the target. Fail hard.
>> +         *
>> +         * TODO: How to handle this in a better way?
>> +         */
>> +        error_report("RAM resized during postcopy.");
>> +        exit(-1);
> 
> Now I'm rethinking the postcopy case....
> 
> ram_dirty_bitmap_reload() should only happen during the postcopy
> recovery, and when that happens the VM should be stopped on both
> sides.  Which means, ram resizing should not trigger there...

But that guest got the chance to run for a bit and eventually reboot
AFAIK. Also, there are other data races possible when used_length
suddenly changes, this is just the most obvious one where things will;
get screwed up.

-- 
Thanks,

David / dhildenb