Re: [PATCH v1 05/13] migrate/ram: Handle RAM block resizes during precopy

[David Hildenbrand <david@redhat.com>]

On 21.02.20 16:14, Dr. David Alan Gilbert wrote:
> * David Hildenbrand (david@redhat.com) wrote:
>> Resizing while migrating is dangerous and does not work as expected.
>> The whole migration code works on the usable_length of ram blocks and does
>> not expect this to change at random points in time.
>>
>> In the case of precopy, the ram block size must not change on the source,
>> after syncing the RAM block list in ram_save_setup(), so as long as the
>> guest is still running on the source.
>>
>> Resizing can be trigger *after* (but not during) a reset in
>> ACPI code by the guest
>> - hw/arm/virt-acpi-build.c:acpi_ram_update()
>> - hw/i386/acpi-build.c:acpi_ram_update()
>>
>> Use the ram block notifier to get notified about resizes. Let's simply
>> cancel migration and indicate the reason. We'll continue running on the
>> source. No harm done.
>>
>> Update the documentation. Postcopy will be handled separately.
>>
>> Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
>> Cc: Juan Quintela <quintela@redhat.com>
>> Cc: Eduardo Habkost <ehabkost@redhat.com>
>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>> Cc: Igor Mammedov <imammedo@redhat.com>
>> Cc: "Michael S. Tsirkin" <mst@redhat.com>
>> Cc: Richard Henderson <richard.henderson@linaro.org>
>> Cc: Shannon Zhao <shannon.zhao@linaro.org>
>> Cc: Alex Bennée <alex.bennee@linaro.org>
>> Cc: Peter Xu <peterx@redhat.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>  exec.c                |  5 +++--
>>  include/exec/memory.h | 10 ++++++----
>>  migration/migration.c |  9 +++++++--
>>  migration/migration.h |  1 +
>>  migration/ram.c       | 41 +++++++++++++++++++++++++++++++++++++++++
>>  5 files changed, 58 insertions(+), 8 deletions(-)
>>
>> diff --git a/exec.c b/exec.c
>> index b75250e773..8b015821d6 100644
>> --- a/exec.c
>> +++ b/exec.c
>> @@ -2120,8 +2120,9 @@ static int memory_try_enable_merging(void *addr, size_t len)
>>      return qemu_madvise(addr, len, QEMU_MADV_MERGEABLE);
>>  }
>>  
>> -/* Only legal before guest might have detected the memory size: e.g. on
>> - * incoming migration, or right after reset.
>> +/*
>> + * Resizing RAM while migrating can result in the migration being canceled.
>> + * Care has to be taken if the guest might have already detected the memory.
>>   *
>>   * As memory core doesn't know how is memory accessed, it is up to
>>   * resize callback to update device state and/or add assertions to detect
>> diff --git a/include/exec/memory.h b/include/exec/memory.h
>> index e85b7de99a..de111347e8 100644
>> --- a/include/exec/memory.h
>> +++ b/include/exec/memory.h
>> @@ -113,7 +113,7 @@ typedef struct IOMMUNotifier IOMMUNotifier;
>>  #define RAM_SHARED     (1 << 1)
>>  
>>  /* Only a portion of RAM (used_length) is actually used, and migrated.
>> - * This used_length size can change across reboots.
>> + * Resizing RAM while migrating can result in the migration being canceled.
>>   */
>>  #define RAM_RESIZEABLE (1 << 2)
>>  
>> @@ -843,7 +843,9 @@ void memory_region_init_ram_shared_nomigrate(MemoryRegion *mr,
>>   *                                     RAM.  Accesses into the region will
>>   *                                     modify memory directly.  Only an initial
>>   *                                     portion of this RAM is actually used.
>> - *                                     The used size can change across reboots.
>> + *                                     Changing the size while migrating
>> + *                                     can result in the migration being
>> + *                                     canceled.
>>   *
>>   * @mr: the #MemoryRegion to be initialized.
>>   * @owner: the object that tracks the region's reference count
>> @@ -1464,8 +1466,8 @@ void *memory_region_get_ram_ptr(MemoryRegion *mr);
>>  
>>  /* memory_region_ram_resize: Resize a RAM region.
>>   *
>> - * Only legal before guest might have detected the memory size: e.g. on
>> - * incoming migration, or right after reset.
>> + * Resizing RAM while migrating can result in the migration being canceled.
>> + * Care has to be taken if the guest might have already detected the memory.
>>   *
>>   * @mr: a memory region created with @memory_region_init_resizeable_ram.
>>   * @newsize: the new size the region
>> diff --git a/migration/migration.c b/migration/migration.c
>> index 8fb68795dc..ac9751dbe5 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..57f32011a3 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 +3711,48 @@ 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)
>> +{
>> +    ram_addr_t offset;
>> +    Error *err = NULL;
>> +    RAMBlock *rb = qemu_ram_block_from_host(host, false, &offset);
>> +
>> +    if (ramblock_is_ignored(rb)) {
>> +        return;
>> +    }
>> +
>> +    /*
>> +     * Some resizes are triggered on the migration target by precopy code,
>> +     * when synchronizing RAM block sizes. In these cases, the VM is not
>> +     * running and migration is not idle. We have to ignore these resizes,
>> +     * as we only care about resizes during precopy on the migration source.
>> +     * This handler is always registered, so ignore when migration is idle.
>> +     */
>> +    if (migration_is_idle() || !runstate_is_running() ||
>> +        postcopy_is_running()) {
>> +        return;
>> +    }
>> +
>> +    /*
>> +     * 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 block '%s' resized during precopy.", rb->idstr);
>> +    migrate_set_error(migrate_get_current(), err);
>> +    error_free(err);
>> +    migration_cancel();
>> +}
>> +
>> +static RAMBlockNotifier ram_mig_ram_notifier = {
>> +    .ram_block_resized = ram_mig_ram_block_resized,
>> +};
>> +
>>  void ram_mig_init(void)
>>  {
>>      qemu_mutex_init(&XBZRLE.lock);
>>      register_savevm_live("ram", 0, 4, &savevm_ram_handlers, &ram_state);
>> +    ram_block_notifier_add(&ram_mig_ram_notifier);
> 
> Can we avoid the question of the 'is_idle' checks by doing this
> registration in save_setup/load_setup and unregistering in
> save_cleanup/load_cleanup?

Well, I figured it out now :)

migration_is_idle() is enough to handle it in this patch.

> 
> That means if we land in the handler we know we're in either an incoming
> or outgoing migration and then you just have to check which?

Can save/load race with other QEMU code that might register/unregister
notifiers? I want to avoid having to introduce locking just for that.

-- 
Thanks,

David / dhildenb