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

[David Hildenbrand]

On 21.02.20 10:18, David Hildenbrand wrote:
> On 20.02.20 21:17, Peter Xu wrote:
>> On Thu, Feb 20, 2020 at 04:16:02PM +0100, David Hildenbrand wrote:
>>> On 19.02.20 17:17, David Hildenbrand 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() ||
>>
>> So I noticed that I mis-misread the code after chat with Dave...
>>
>> migration_is_idle() should only return false if on the source and only
>> if during migration.  Destination should still return true for that
>> (destination VM reads state from MigrationIncomingState.state
>> instead).
>>
>> With that, I think we can drop the confusing !runstate_is_running()
>> check because migration_is_idle() will cover that (and touch up the
>> comment too)?
> 
> So, we want to cancel migration whenever we are on the source and we are
> migrating (postcopy). Resizing will only happen while the VM is running
> on the source once we're migrating.
> 
> So you're saying that
> 
> if (migration_is_idle() || postcopy_is_running()) {
>       return:
> }
> 
> is enough. Will migration_is_idle() always return true on the
> destination (I remember something different, but might be *I* misread
> the code)?
> 
> Then this would distill down to
> 
> if (migration_is_idle()) {
>       return:
> }
> 
> Which would be nice.

... but looking at process_incoming_migration_co(), we'll set the state
to MIGRATION_STATUS_ACTIVE right at the start.


So dropping postcopy_is_running() would be wrong and dropping
!runstate_is_running() would be wrong as well right now.

What we actually want is

if (!migration_outgoing()) {
        return;
}

Any experts?

-- 
Thanks,

David / dhildenb