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

[Dr. David Alan Gilbert]

* David Hildenbrand (address@hidden) 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" <address@hidden>
> Cc: Juan Quintela <address@hidden>
> Cc: Eduardo Habkost <address@hidden>
> Cc: Paolo Bonzini <address@hidden>
> Cc: Igor Mammedov <address@hidden>
> Cc: "Michael S. Tsirkin" <address@hidden>
> Cc: Richard Henderson <address@hidden>
> Cc: Shannon Zhao <address@hidden>
> Cc: Alex Bennée <address@hidden>
> Cc: Peter Xu <address@hidden>
> Signed-off-by: David Hildenbrand <address@hidden>
> ---
>  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?

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?

Dave

>  }
> -- 
> 2.24.1
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK