Re: [Qemu-devel] [PATCH v4] savevm: Really verify if a drive supports snapshots

[MORITA Kazutaka]

At Fri,  4 Jun 2010 16:35:59 -0300,
Miguel Di Ciurcio Filho wrote:
> 
> 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() actually do
> what it must do and enforces better tests to avoid errors in the middle of
> do_savevm(). bdrv_has_snapshot() is removed and replaced by 
> bdrv_can_snapshot()
> where appropriate.
> 
> bdrv_can_snapshot() was moved from savevm.c to block.c. It makes more sense 
> to me.
> 
> 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 <address@hidden>
> ---
>  block.c  |   11 +++++++++++
>  block.h  |    1 +
>  savevm.c |   58 ++++++++++++++++++++++++++++++++++++----------------------
>  3 files changed, 48 insertions(+), 22 deletions(-)
> 
> diff --git a/block.c b/block.c
> index cd70730..ace3cdb 100644
> --- a/block.c
> +++ b/block.c
> @@ -1720,6 +1720,17 @@ void bdrv_debug_event(BlockDriverState *bs, 
> BlkDebugEvent event)
>  /**************************************************************/
>  /* handling of snapshots */
>  
> +int bdrv_can_snapshot(BlockDriverState *bs)
> +{
> +    BlockDriver *drv = bs->drv;
> +    if (!drv || !drv->bdrv_snapshot_create || bdrv_is_removable(bs) ||
> +        bdrv_is_read_only(bs)) {
> +        return 0;
> +    }
> +
> +    return 1;
> +}
> +

The underlying protocol could support snapshots, so I think we should
check against bs->file too.

--- a/block.c
+++ b/block.c
@@ -1671,6 +1671,9 @@ int bdrv_can_snapshot(BlockDriverState *bs)
     BlockDriver *drv = bs->drv;
     if (!drv || !drv->bdrv_snapshot_create || bdrv_is_removable(bs) ||
         bdrv_is_read_only(bs)) {
+        if (bs->file) {
+            return bdrv_can_snapshot(bs->file);
+        }
         return 0;
     }
 
Regards,

Kazutaka