[Qemu-devel] Re: [PATCH] savevm: Really verify if a drive supports snapshots

[Kevin Wolf <kwolf@redhat.com>]

Am 28.05.2010 20:18, schrieb Miguel Di Ciurcio Filho:
> Both bdrv_can_snapshot() and bdrv_has_snapshot() does not work as advertized.
> 
> First issue: Their names implies different porpouses, but they do the same thing
> and have exactly the same code. Maybe copied and pasted and forgotten?
> bdrv_has_snapshot() is called in various places for actually checking if there
> is snapshots or not.
> 
> Second issue: the way bdrv_can_snapshot() verifies if a block driver supports or
> not snapshots does not catch all cases. E.g.: a raw image.
> 
> So when do_savevm() is called, first thing it does is to set a global
> BlockDriverState to save the VM memory state calling get_bs_snapshots().
> 
> static BlockDriverState *get_bs_snapshots(void)
> {
>     BlockDriverState *bs;
>     DriveInfo *dinfo;
> 
>     if (bs_snapshots)
>         return bs_snapshots;
>     QTAILQ_FOREACH(dinfo, &drives, next) {
>         bs = dinfo->bdrv;
>         if (bdrv_can_snapshot(bs))
>             goto ok;
>     }
>     return NULL;
>  ok:
>     bs_snapshots = bs;
>     return bs;
> }
> 
> bdrv_can_snapshot() may return a BlockDriverState that does not support
> snapshots and do_savevm() goes on.
> 
> Later on in do_savevm(), we find:
> 
>     QTAILQ_FOREACH(dinfo, &drives, next) {
>         bs1 = dinfo->bdrv;
>         if (bdrv_has_snapshot(bs1)) {
>             /* Write VM state size only to the image that contains the state */
>             sn->vm_state_size = (bs == bs1 ? vm_state_size : 0);
>             ret = bdrv_snapshot_create(bs1, sn);
>             if (ret < 0) {
>                 monitor_printf(mon, "Error while creating snapshot on '%s'\n",
>                                bdrv_get_device_name(bs1));
>             }
>         }
>     }
> 
> bdrv_has_snapshot(bs1) is not checking if the device does support or has
> snapshots as explained above. Only in bdrv_snapshot_create() the device is
> actually checked for snapshot support.
> 
> So, in cases where the first device supports snapshots, and the second does not,
> the snapshot on the first will happen anyways. I believe this is not a good
> behavior. It should be an all or nothing process.
> 
> This patch addresses these issues by making bdrv_can_snapshot() and
> bdrv_has_snapshot() actually do what they must do and enforces better tests to
> avoid errors in the middle of do_savevm().
> 
> The functions were moved from savevm.c to block.c. It makes more sense to me.
> 
> The bdrv_has_snapshot() is not beaultiful, but it does the job. I think having
> this function avaible in the BlockDriver would be the best option.
> 
> The loadvm_state() function was updated too to enforce that when loading a VM at
> least all writable devices must support snapshots too.
> 
> Signed-off-by: Miguel Di Ciurcio Filho <miguel.filho@gmail.com>

Markus, I think this implements mostly what we discussed the other day.
Not sure if you already have a patch for doing this - if so, maybe could
compare the patches and give it a review this way?

I seem to remember that we came to the conclusion that
bdrv_has_snapshot() isn't needed at all and should be dropped. Any user
should be using bdrv_can_snapshot() instead as this is what they really
want.

> ---
>  block.c  |   47 ++++++++++++++++++++++++++++++++++++++++++-----
>  block.h  |    2 ++
>  savevm.c |   48 +++++++++++++++++++++++++-----------------------
>  3 files changed, 69 insertions(+), 28 deletions(-)
> 
> diff --git a/block.c b/block.c
> index cd70730..7eddc15 100644
> --- a/block.c
> +++ b/block.c
> @@ -1720,15 +1720,52 @@ void bdrv_debug_event(BlockDriverState *bs, BlkDebugEvent event)
>  /**************************************************************/
>  /* handling of snapshots */
>  
> -int bdrv_snapshot_create(BlockDriverState *bs,
> -                         QEMUSnapshotInfo *sn_info)
> +int bdrv_can_snapshot(BlockDriverState *bs)
>  {
>      BlockDriver *drv = bs->drv;
> -    if (!drv)
> +    if (!drv) {
> +        return -ENOMEDIUM;
> +    }
> +
> +    if (!drv->bdrv_snapshot_create || bdrv_is_removable(bs) ||
> +        bdrv_is_read_only(bs)) {
> +        return -ENOTSUP;
> +    }
> +
> +    return 1;
> +}

Returning either 1 or -errno is a strange interface. I'm not sure which
of 1/0 or 0/-errno is better in this case, but I'd suggest to take one
of these.

> +int bdrv_has_snapshot(BlockDriverState *bs)
> +{
> +    int ret;
> +    QEMUSnapshotInfo *sn_tab;
> +    BlockDriver *drv = bs->drv;
> +    if (!drv) {
>          return -ENOMEDIUM;
> -    if (!drv->bdrv_snapshot_create)
> +    }
> +
> +    if (!drv->bdrv_snapshot_list) {
>          return -ENOTSUP;
> -    return drv->bdrv_snapshot_create(bs, sn_info);
> +    }
> +
> +    ret = drv->bdrv_snapshot_list(bs, &sn_tab);
> +
> +    if (sn_tab) {
> +        qemu_free(sn_tab);
> +    }
> +
> +    return ret;
> +}
> +
> +int bdrv_snapshot_create(BlockDriverState *bs,
> +                         QEMUSnapshotInfo *sn_info)
> +{
> +    BlockDriver *drv = bs->drv;
> +    if (bdrv_can_snapshot(bs) > 0) {
> +        return drv->bdrv_snapshot_create(bs, sn_info);
> +    }
> +
> +    return -1;
>  }
>  
>  int bdrv_snapshot_goto(BlockDriverState *bs,
> diff --git a/block.h b/block.h
> index 24efeb6..c536f1c 100644
> --- a/block.h
> +++ b/block.h
> @@ -173,6 +173,8 @@ int bdrv_get_info(BlockDriverState *bs, BlockDriverInfo *bdi);
>  const char *bdrv_get_encrypted_filename(BlockDriverState *bs);
>  void bdrv_get_backing_filename(BlockDriverState *bs,
>                                 char *filename, int filename_size);
> +int bdrv_can_snapshot(BlockDriverState *bs);
> +int bdrv_has_snapshot(BlockDriverState *bs);
>  int bdrv_snapshot_create(BlockDriverState *bs,
>                           QEMUSnapshotInfo *sn_info);
>  int bdrv_snapshot_goto(BlockDriverState *bs,
> diff --git a/savevm.c b/savevm.c
> index dc20390..9bc232f 100644
> --- a/savevm.c
> +++ b/savevm.c
> @@ -1574,22 +1574,6 @@ out:
>      return ret;
>  }
>  
> -/* device can contain snapshots */
> -static int bdrv_can_snapshot(BlockDriverState *bs)
> -{
> -    return (bs &&
> -            !bdrv_is_removable(bs) &&
> -            !bdrv_is_read_only(bs));
> -}
> -
> -/* device must be snapshots in order to have a reliable snapshot */
> -static int bdrv_has_snapshot(BlockDriverState *bs)
> -{
> -    return (bs &&
> -            !bdrv_is_removable(bs) &&
> -            !bdrv_is_read_only(bs));
> -}
> -
>  static BlockDriverState *get_bs_snapshots(void)
>  {
>      BlockDriverState *bs;
> @@ -1599,7 +1583,7 @@ static BlockDriverState *get_bs_snapshots(void)
>          return bs_snapshots;
>      QTAILQ_FOREACH(dinfo, &drives, next) {
>          bs = dinfo->bdrv;
> -        if (bdrv_can_snapshot(bs))
> +        if (bdrv_can_snapshot(bs) > 0)
>              goto ok;
>      }
>      return NULL;
> @@ -1642,7 +1626,7 @@ static int del_existing_snapshots(Monitor *mon, const char *name)
>  
>      QTAILQ_FOREACH(dinfo, &drives, next) {
>          bs = dinfo->bdrv;
> -        if (bdrv_can_snapshot(bs) &&
> +        if ((bdrv_can_snapshot(bs) > 0) &&
>              bdrv_snapshot_find(bs, snapshot, name) >= 0)
>          {
>              ret = bdrv_snapshot_delete(bs, name);
> @@ -1674,12 +1658,30 @@ void do_savevm(Monitor *mon, const QDict *qdict)
>  #endif
>      const char *name = qdict_get_try_str(qdict, "name");
>  
> +    /* Verify if there is have a device that doesn't support snapshots and is writable*/
> +    ret = 0;
> +    QTAILQ_FOREACH(dinfo, &drives, next) {
> +        bs = dinfo->bdrv;
> +
> +        if (bdrv_is_removable(bs) || bdrv_is_read_only(bs)) {
> +            continue;
> +        }
> +
> +        if ((ret = bdrv_can_snapshot(bs)) < 0) {

This may overwrite the error of the previous loop iteration with a
success return value. It's probably not what you want.

Other than that it looks good to me.

Kevin