On Tue, Sep 17, 2024 at 03:58:02PM -0300, Fabiano Rosas wrote:
> Fix a segmentation fault in multifd when rb->receivedmap is cleared
> too early.
>
> After commit 5ef7e26bdb ("migration/multifd: solve zero page causing
> multiple page faults"), multifd started using the rb->receivedmap
> bitmap, which belongs to ram.c and is initialized and *freed* from the
> ram SaveVMHandlers.
>
> Multifd threads are live until migration_incoming_state_destroy(),
> which is called after qemu_loadvm_state_cleanup(), leading to a crash
> when accessing rb->receivedmap.
>
> process_incoming_migration_co() ...
> qemu_loadvm_state() multifd_nocomp_recv()
> qemu_loadvm_state_cleanup() ramblock_recv_bitmap_set_offset()
> rb->receivedmap = NULL set_bit_atomic(...,
> rb->receivedmap)
> ...
> migration_incoming_state_destroy()
> multifd_recv_cleanup()
> multifd_recv_terminate_threads(NULL)
>
> Move the loadvm cleanup into migration_incoming_state_destroy(), after
> multifd_recv_cleanup() to ensure multifd threads have already exited
> when rb->receivedmap is cleared.
>
> Adjust the postcopy listen thread comment to indicate that we still
> want to skip the cpu synchronization.
>
> CC: qemu-sta...@nongnu.org
> Fixes: 5ef7e26bdb ("migration/multifd: solve zero page causing multiple page
> faults")
> Signed-off-by: Fabiano Rosas <faro...@suse.de>
Reviewed-by: Peter Xu <pet...@redhat.com>
One trivial question below..
> ---
> migration/migration.c | 1 +
> migration/savevm.c | 6 ++++--
> 2 files changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/migration/migration.c b/migration/migration.c
> index 3dea06d577..b190a574b1 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -378,6 +378,7 @@ void migration_incoming_state_destroy(void)
> struct MigrationIncomingState *mis = migration_incoming_get_current();
>
> multifd_recv_cleanup();
Would you mind I add a comment squashed here when queue?
/*
* RAM state cleanup needs to happen after multifd cleanup, because
* multifd threads can use some of its states (receivedmap).
*/
> + qemu_loadvm_state_cleanup();
>
> if (mis->to_src_file) {
> /* Tell source that we are done */
> diff --git a/migration/savevm.c b/migration/savevm.c
> index d0759694fd..7e1e27182a 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -2979,7 +2979,10 @@ int qemu_loadvm_state(QEMUFile *f)
> trace_qemu_loadvm_state_post_main(ret);
>
> if (mis->have_listen_thread) {
> - /* Listen thread still going, can't clean up yet */
> + /*
> + * Postcopy listen thread still going, don't synchronize the
> + * cpus yet.
> + */
> return ret;
> }
>
> @@ -3022,7 +3025,6 @@ int qemu_loadvm_state(QEMUFile *f)
> }
> }
>
> - qemu_loadvm_state_cleanup();
> cpu_synchronize_all_post_init();
>
> return ret;
> --
> 2.35.3
>
--
Peter Xu
