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

[David Hildenbrand <david@redhat.com>]

On 21.02.20 11:10, David Hildenbrand wrote:
> 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" <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() ||
>>>
>>> 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?
> 

Lol, man this code is confusing.

So, migration_is_idle() really only checks current_migration, not
current_incoming.

I didn't expect to be knees deep in migration code at this point ...

if (migration_is_idle()) {
	return:
}

works. Thanks!

-- 
Thanks,

David / dhildenb