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Re: [PATCH v1 05/13] migrate/ram: Handle RAM block resizes during precop
From: |
David Hildenbrand |
Subject: |
Re: [PATCH v1 05/13] migrate/ram: Handle RAM block resizes during precopy |
Date: |
Fri, 21 Feb 2020 11:10:59 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 |
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
Re: [PATCH v1 05/13] migrate/ram: Handle RAM block resizes during precopy, Dr. David Alan Gilbert, 2020/02/21
[PATCH v1 08/13] migrate/ram: Simplify host page handling in ram_load_postcopy(), David Hildenbrand, 2020/02/19
[PATCH v1 09/13] migrate/ram: Consolidate variable reset after placement in ram_load_postcopy(), David Hildenbrand, 2020/02/19
[PATCH v1 07/13] migrate/ram: Get rid of "place_source" in ram_load_postcopy(), David Hildenbrand, 2020/02/19