Am 04.06.2010 21:35, 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() 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 <miguel.fi...@gmail.com>
Thanks, applied to the block branch.
Kevin
