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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* David Hildenbrand (david@redhat.com) wrote:
> 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.

<looks at code> It looks like it can; qemu_savevm_state_setup
is called from near the top of migration_thread, so I don't
think it's holding locks at that point; and all the existing notifier
changes on this notifier seem to be done from either init or vfio_open
which I guess is under the big lock (?)

Dave


> -- 
> Thanks,
> 
> David / dhildenb
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK