Re: [PATCH v2 07/19] migration/rdma: Use ram_block_discard_disable()

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

* David Hildenbrand (david@redhat.com) wrote:
> RDMA will pin all guest memory (as documented in docs/rdma.txt). We want
> to disable RAM block discards - however, to keep it simple use
> ram_block_discard_is_required() instead of inhibiting.
> 
> Note: It is not sufficient to limit disabling to pin_all. Even when only
> conditionally pinning 1 MB chunks, as soon as one page within such a
> chunk was discarded and one page not, the discarded pages will be pinned
> as well.
> 
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Juan Quintela <quintela@redhat.com>
> Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

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

> ---
>  migration/rdma.c | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/migration/rdma.c b/migration/rdma.c
> index 967fda5b0c..57e2cbc8ca 100644
> --- a/migration/rdma.c
> +++ b/migration/rdma.c
> @@ -29,6 +29,7 @@
>  #include "qemu/sockets.h"
>  #include "qemu/bitmap.h"
>  #include "qemu/coroutine.h"
> +#include "exec/memory.h"
>  #include <sys/socket.h>
>  #include <netdb.h>
>  #include <arpa/inet.h>
> @@ -4017,8 +4018,14 @@ void rdma_start_incoming_migration(const char *host_port, Error **errp)
>      Error *local_err = NULL;
>  
>      trace_rdma_start_incoming_migration();
> -    rdma = qemu_rdma_data_init(host_port, &local_err);
>  
> +    /* Avoid ram_block_discard_disable(), cannot change during migration. */
> +    if (ram_block_discard_is_required()) {
> +        error_setg(errp, "RDMA: cannot disable RAM discard");
> +        return;
> +    }
> +
> +    rdma = qemu_rdma_data_init(host_port, &local_err);
>      if (rdma == NULL) {
>          goto err;
>      }
> @@ -4065,10 +4072,17 @@ void rdma_start_outgoing_migration(void *opaque,
>                              const char *host_port, Error **errp)
>  {
>      MigrationState *s = opaque;
> -    RDMAContext *rdma = qemu_rdma_data_init(host_port, errp);
>      RDMAContext *rdma_return_path = NULL;
> +    RDMAContext *rdma;
>      int ret = 0;
>  
> +    /* Avoid ram_block_discard_disable(), cannot change during migration. */
> +    if (ram_block_discard_is_required()) {
> +        error_setg(errp, "RDMA: cannot disable RAM discard");
> +        return;
> +    }
> +
> +    rdma = qemu_rdma_data_init(host_port, errp);
>      if (rdma == NULL) {
>          goto err;
>      }
> -- 
> 2.25.4
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK