Re: [Qemu-devel] [PATCH RFC 1/2] snapshot: create helper to test that block drivers supports snapshots

[Juan Quintela <quintela@redhat.com>]

"Denis V. Lunev" <den@openvz.org> wrote:
> The patch enforces proper locking for this operation.
>
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Stefan Hajnoczi <stefanha@redhat.com>
> CC: Juan Quintela <quintela@redhat.com>
> ---
> Patches are compile-tested only. Sent to check the approach, naming and
> functions placement. Functions are returning bad BlockDriver via
> parameter to make clear distinction when I'll have to return BS to write.
>
>  block/snapshot.c         | 35 +++++++++++++++++++++++++++++++++++
>  include/block/snapshot.h |  9 +++++++++
>  migration/savevm.c       | 17 ++++-------------
>  3 files changed, 48 insertions(+), 13 deletions(-)
>
> diff --git a/block/snapshot.c b/block/snapshot.c
> index 89500f2..6b5ce4e 100644
> --- a/block/snapshot.c
> +++ b/block/snapshot.c
> @@ -25,6 +25,7 @@
>  #include "block/snapshot.h"
>  #include "block/block_int.h"
>  #include "qapi/qmp/qerror.h"
> +#include "monitor/monitor.h"

You don't need the monitor here O:-)

>  
>  QemuOptsList internal_snapshot_opts = {
>      .name = "snapshot",
> @@ -356,3 +357,37 @@ int bdrv_snapshot_load_tmp_by_id_or_name(BlockDriverState *bs,
>  
>      return ret;
>  }
> +
> +
> +/* Group operations. All block drivers are involved.
> + * These functions will properly handle dataplace (take aio_context_acquire
> + * when appropriate for appropriate block drivers
> + *
> + * Returned block driver will be always locked.
> + */
> +
> +bool bdrv_snapshot_all_can_snapshot(BlockDriverState **first_bad_bs)

bdrv_snapshot_is_possible???

> +{
> +    BlockDriverState *bs;
> +
> +    while ((bs = bdrv_next(bs))) {
> +        bool ok;
> +        AioContext *ctx = bdrv_get_aio_context(bs);
> +
> +        if (!bdrv_is_inserted(bs) || bdrv_is_read_only(bs)) {
> +            continue;
> +        }
> +
> +        aio_context_acquire(ctx);

I think that you should get the lock before the bdrv_is_inserted, but
who am I to know for sure O:-)

> +        ok = bdrv_can_snapshot(bs);
> +        aio_context_release(ctx);
> +
> +        if (!ok) {
> +            *first_bad_bs = bs;
> +            return false;
> +        }
> +    }
> +
> +    *first_bad_bs = NULL;
> +    return true;
> +}
> diff --git a/include/block/snapshot.h b/include/block/snapshot.h
> index 770d9bb..61b4b5d 100644
> --- a/include/block/snapshot.h
> +++ b/include/block/snapshot.h
> @@ -75,4 +75,13 @@ int bdrv_snapshot_load_tmp(BlockDriverState *bs,
>  int bdrv_snapshot_load_tmp_by_id_or_name(BlockDriverState *bs,
>                                           const char *id_or_name,
>                                           Error **errp);
> +
> +
> +/* Group operations. All block drivers are involved.
> + * These functions will properly handle dataplace (take aio_context_acquire
> + * when appropriate for appropriate block drivers
> + *
> + */
> +bool bdrv_snapshot_all_can_snapshot(BlockDriverState **first_bad_bs);
> +
>  #endif
> diff --git a/migration/savevm.c b/migration/savevm.c
> index dbcc39a..91ba0bf 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -1290,19 +1290,10 @@ void hmp_savevm(Monitor *mon, const QDict *qdict)
>      const char *name = qdict_get_try_str(qdict, "name");
>      Error *local_err = NULL;
>  
> -    /* Verify if there is a device that doesn't support snapshots and is writable */
> -    bs = NULL;
> -    while ((bs = bdrv_next(bs))) {
> -
> -        if (!bdrv_is_inserted(bs) || bdrv_is_read_only(bs)) {
> -            continue;
> -        }
> -
> -        if (!bdrv_can_snapshot(bs)) {
> -            monitor_printf(mon, "Device '%s' is writable but does not support snapshots.\n",
> -                               bdrv_get_device_name(bs));
> -            return;
> -        }
> +    if (bdrv_snapshot_all_can_snapshot(&bs)) {
> +        monitor_printf(mon, "Device '%s' is writable but does not "
> +                       "support snapshots.\n", bdrv_get_device_name(bs));
> +        return;
>      }
>  
>      bs = find_vmstate_bs();

ok with the savevm.c changes.