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migration: Pre-fault memory before starting background snasphot

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This commit solves the issue with userfault_fd WP feature that
background snapshot is based on. For any never poluated or discarded
memory page, the UFFDIO_WRITEPROTECT ioctl() would skip updating
PTE for that page, thereby loosing WP setting for it.

So we need to pre-fault pages for each RAM block to be protected
before making a userfault_fd wr-protect ioctl().

Fixes: 278e2f551a (migration: support
  UFFD write fault processing in ram_save_iterate())
Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
Reported-by: David Hildenbrand <david@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210401092226.102804-4-andrey.gruzdev@virtuozzo.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
dgilbert:
  Bodged ifdef __linux__ on ram_write_tracking_prepare, should really
      go in a stub
This commit is contained in:
Andrey Gruzdev 2021-04-01 12:22:25 +03:00 • committed by Dr. David Alan Gilbert
parent 1a8e44a89f
commit eeccb99c9d
3 changed files with 58 additions and 0 deletions
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8
migration/migration.c
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@ -3827,6 +3827,14 @@ static void *bg_migration_thread(void *opaque)
update_iteration_initial_status(s);
/*
* Prepare for tracking memory writes with UFFD-WP - populate
* RAM pages before protecting.
*/
#ifdef __linux__
ram_write_tracking_prepare();
#endif
qemu_savevm_state_header(s->to_dst_file);
qemu_savevm_state_setup(s->to_dst_file);