Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating

[Peter Xu <peterx@redhat.com>]

On Fri, Feb 14, 2020 at 06:32:35PM +0100, David Hildenbrand wrote:
> On 14.02.20 18:29, Peter Xu wrote:
> > On Fri, Feb 14, 2020 at 01:02:46PM +0100, David Hildenbrand wrote:
> >> From c0049ac2e95d6756037db918852c507fb88297d9 Mon Sep 17 00:00:00 2001
> >> From: David Hildenbrand <david@redhat.com>
> >> Date: Fri, 14 Feb 2020 13:01:03 +0100
> >> Subject: [PATCH v1] tmp
> >>
> >> Signed-off-by: David Hildenbrand <david@redhat.com>
> >> ---
> >>  migration/migration.c |  9 +++++++--
> >>  migration/migration.h |  1 +
> >>  migration/ram.c       | 42 ++++++++++++++++++++++++++++++++++++++++++
> >>  3 files changed, 50 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/migration/migration.c b/migration/migration.c
> >> index 3a21a4686c..0e7efe2920 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..f86f32b453 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 +3711,49 @@ 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)
> >> +{
> >> +    /*
> >> +     * We don't care about resizes triggered on incoming migration (when
> >> +     * syncing ram blocks), or of course, when no migration is going on.
> >> +     */
> >> +    if (migration_is_idle() || !runstate_is_running()) {
> >> +        return;
> >> +    }
> > 
> > I feel like migration_is_idle() check is enough.  Firstly, I feel like
> > we allow migration even with VM stopped.  At the meantime, if VM is
> > not running, I see no reason that this resizing will happen after all? :)
> 
> Migration code resizes ram blocks when synchronizing the ram state. See
> qemu_ram_resize() in ram_load_precopy()
> 
> That happens on incoming migration while the vm is stopped.

Ah so your comment is for that which I misread.  I'm surprised even
the incoming migration will set MigrationState and migration_is_idle()
will return false for it, since we've got MigrationIncomingState after
all.  But yeh that seems to be correct.

Then it seems fine.  It's just a bit unclear even with the comment.
Another alternative is we only register this resize() hook when
migration starts, and unregister it when stopped.  Then we can even
drop the migration_is_idle().

> 
> > 
> >> +
> >> +    if (!postcopy_is_running()) {
> >> +        Error *err = NULL;
> >> +
> >> +        /*
> >> +         * 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 resized during precopy.");
> >> +        migrate_set_error(migrate_get_current(), err);
> >> +        error_free(err);
> >> +        migration_cancel();
> >> +    } else {
> >> +        /*
> >> +         * Postcopy code cannot deal with the size of ram blocks changing at
> >> +         * random points in time. We're running on the target. Fail hard.
> >> +         *
> >> +         * TODO: How to handle this in a better way?
> >> +         */
> >> +        error_report("RAM resized during postcopy.");
> >> +        exit(-1);
> > 
> > Now I'm rethinking the postcopy case....
> > 
> > ram_dirty_bitmap_reload() should only happen during the postcopy
> > recovery, and when that happens the VM should be stopped on both
> > sides.  Which means, ram resizing should not trigger there...
> 
> But that guest got the chance to run for a bit and eventually reboot
> AFAIK. Also, there are other data races possible when used_length
> suddenly changes, this is just the most obvious one where things will;
> get screwed up.

Right, the major one could be in ram_load_postcopy() where we call
host_from_ram_block_offset().  However if FW extension is the major
use case then it seems to still work (still better than crashing,
isn't it? :).  Or could you let me know where else did I overlook?

Thanks,

-- 
Peter Xu