Re: [PATCH 10/20] migration: Enlarge postcopy recovery to capture !-EIO too

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Xu (peterx@redhat.com) wrote:
> We used to have quite a few places making sure -EIO happened and that's the
> only way to trigger postcopy recovery.  That's based on the assumption that
> we'll only return -EIO for channel issues.
> 
> It'll work in 99.99% cases but logically that won't cover some corner cases.
> One example is e.g. ram_block_from_stream() could fail with an interrupted
> network, then -EINVAL will be returned instead of -EIO.
> 
> I remembered Dave Gilbert pointed that out before, but somehow this is
> overlooked.  Neither did I encounter anything outside the -EIO error.
> 
> However we'd better touch that up before it triggers a rare VM data loss during
> live migrating.
> 
> To cover as much those cases as possible, remove the -EIO restriction on
> triggering the postcopy recovery, because even if it's not a channel failure,
> we can't do anything better than halting QEMU anyway - the corpse of the
> process may even be used by a good hand to dig out useful memory regions, or
> the admin could simply kill the process later on.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
>  migration/migration.c    | 4 ++--
>  migration/postcopy-ram.c | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index 6e4cc9cc87..67520d3105 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -2877,7 +2877,7 @@ retry:
>  out:
>      res = qemu_file_get_error(rp);
>      if (res) {
> -        if (res == -EIO && migration_in_postcopy()) {
> +        if (res && migration_in_postcopy()) {
>              /*
>               * Maybe there is something we can do: it looks like a
>               * network down issue, and we pause for a recovery.
> @@ -3478,7 +3478,7 @@ static MigThrError migration_detect_error(MigrationState *s)
>          error_free(local_error);
>      }
>  
> -    if (state == MIGRATION_STATUS_POSTCOPY_ACTIVE && ret == -EIO) {
> +    if (state == MIGRATION_STATUS_POSTCOPY_ACTIVE && ret) {
>          /*
>           * For postcopy, we allow the network to be down for a
>           * while. After that, it can be continued by a
> diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> index d3ec22e6de..6be510fea4 100644
> --- a/migration/postcopy-ram.c
> +++ b/migration/postcopy-ram.c
> @@ -1038,7 +1038,7 @@ retry:
>                                          msg.arg.pagefault.address);
>              if (ret) {
>                  /* May be network failure, try to wait for recovery */
> -                if (ret == -EIO && postcopy_pause_fault_thread(mis)) {
> +                if (postcopy_pause_fault_thread(mis)) {
>                      /* We got reconnected somehow, try to continue */
>                      goto retry;
>                  } else {
> -- 
> 2.32.0
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK