[Qemu-devel] [PATCH v2] savevm: Really verify if a drive supports snapshots

[Qemu-devel] [PATCH v2] savevm: Really verify if a drive supports snapshots

[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.filho@gmail.com>
---
 block.c   |   21 ++++++++++++++++-----
 block.h   |    1 +
 monitor.c |    5 ++++-
 savevm.c  |   58 ++++++++++++++++++++++++++++++++++++----------------------
 4 files changed, 57 insertions(+), 28 deletions(-)

diff --git a/block.c b/block.c
index cd70730..1115a60 100644
--- a/block.c
+++ b/block.c
@@ -1720,15 +1720,26 @@ 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;
+}
+
 int bdrv_snapshot_create(BlockDriverState *bs,
                          QEMUSnapshotInfo *sn_info)
 {
     BlockDriver *drv = bs->drv;
-    if (!drv)
-        return -ENOMEDIUM;
-    if (!drv->bdrv_snapshot_create)
-        return -ENOTSUP;
-    return drv->bdrv_snapshot_create(bs, sn_info);
+    if (bdrv_can_snapshot(bs)) {
+        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..fbcd8af 100644
--- a/block.h
+++ b/block.h
@@ -173,6 +173,7 @@ 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_snapshot_create(BlockDriverState *bs,
                          QEMUSnapshotInfo *sn_info);
 int bdrv_snapshot_goto(BlockDriverState *bs,
diff --git a/monitor.c b/monitor.c
index ad50f12..8b4a1d7 100644
--- a/monitor.c
+++ b/monitor.c
@@ -2468,8 +2468,11 @@ static void do_loadvm(Monitor *mon, const QDict *qdict)
 
     vm_stop(0);
 
-    if (load_vmstate(name) >= 0 && saved_vm_running)
+    if (load_vmstate(name) >= 0 && saved_vm_running) {
         vm_start();
+    } else {
+        monitor_printf(mon, "Failed to load VM state. VM stopped.\n");
+    }
 }
 
 int monitor_get_fd(Monitor *mon, const char *fdname)
diff --git a/savevm.c b/savevm.c
index dc20390..6549ca7 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;
@@ -1599,8 +1583,9 @@ 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)) {
             goto ok;
+        }
     }
     return NULL;
  ok:
@@ -1674,12 +1659,26 @@ void do_savevm(Monitor *mon, const QDict *qdict)
 #endif
     const char *name = qdict_get_try_str(qdict, "name");
 
+    /* Verify if there is a device that doesn't support snapshots and is writable */
+    QTAILQ_FOREACH(dinfo, &drives, next) {
+        bs = dinfo->bdrv;
+
+        if (bdrv_is_removable(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));
+            goto the_end;
+        }
+    }
+
     bs = get_bs_snapshots();
     if (!bs) {
         monitor_printf(mon, "No block device can accept snapshots\n");
         return;
     }
-
     /* ??? Should this occur after vm_stop?  */
     qemu_aio_flush();
 
@@ -1732,7 +1731,7 @@ void do_savevm(Monitor *mon, const QDict *qdict)
 
     QTAILQ_FOREACH(dinfo, &drives, next) {
         bs1 = dinfo->bdrv;
-        if (bdrv_has_snapshot(bs1)) {
+        if (bdrv_can_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);
@@ -1756,6 +1755,21 @@ int load_vmstate(const char *name)
     QEMUFile *f;
     int ret;
 
+    /* Verify if there is a device that doesn't support snapshots and is writable */
+    QTAILQ_FOREACH(dinfo, &drives, next) {
+        bs = dinfo->bdrv;
+
+        if (bdrv_is_removable(bs) || bdrv_is_read_only(bs)) {
+            continue;
+        }
+
+        if (!bdrv_can_snapshot(bs)) {
+            error_report("Device '%s' is writable but does not support snapshots.",
+                               bdrv_get_device_name(bs));
+            return -ENOTSUP;
+        }
+    }
+
     bs = get_bs_snapshots();
     if (!bs) {
         error_report("No block device supports snapshots");
@@ -1767,7 +1781,7 @@ int load_vmstate(const char *name)
 
     QTAILQ_FOREACH(dinfo, &drives, next) {
         bs1 = dinfo->bdrv;
-        if (bdrv_has_snapshot(bs1)) {
+        if (bdrv_can_snapshot(bs1)) {
             ret = bdrv_snapshot_goto(bs1, name);
             if (ret < 0) {
                 switch(ret) {
@@ -1829,7 +1843,7 @@ void do_delvm(Monitor *mon, const QDict *qdict)
 
     QTAILQ_FOREACH(dinfo, &drives, next) {
         bs1 = dinfo->bdrv;
-        if (bdrv_has_snapshot(bs1)) {
+        if (bdrv_can_snapshot(bs1)) {
             ret = bdrv_snapshot_delete(bs1, name);
             if (ret < 0) {
                 if (ret == -ENOTSUP)
@@ -1860,7 +1874,7 @@ void do_info_snapshots(Monitor *mon)
     monitor_printf(mon, "Snapshot devices:");
     QTAILQ_FOREACH(dinfo, &drives, next) {
         bs1 = dinfo->bdrv;
-        if (bdrv_has_snapshot(bs1)) {
+        if (bdrv_can_snapshot(bs1)) {
             if (bs == bs1)
                 monitor_printf(mon, " %s", bdrv_get_device_name(bs1));
         }
-- 
1.7.1

Re: [Qemu-devel] [PATCH v2] savevm: Really verify if a drive supports snapshots

[Kevin Wolf]

Am 29.05.2010 21:55, 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.filho@gmail.com>
> ---
>  block.c   |   21 ++++++++++++++++-----
>  block.h   |    1 +
>  monitor.c |    5 ++++-
>  savevm.c  |   58 ++++++++++++++++++++++++++++++++++++----------------------
>  4 files changed, 57 insertions(+), 28 deletions(-)
> 
> diff --git a/block.c b/block.c
> index cd70730..1115a60 100644
> --- a/block.c
> +++ b/block.c
> @@ -1720,15 +1720,26 @@ 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;
> +}
> +
>  int bdrv_snapshot_create(BlockDriverState *bs,
>                           QEMUSnapshotInfo *sn_info)
>  {
>      BlockDriver *drv = bs->drv;
> -    if (!drv)
> -        return -ENOMEDIUM;
> -    if (!drv->bdrv_snapshot_create)
> -        return -ENOTSUP;
> -    return drv->bdrv_snapshot_create(bs, sn_info);
> +    if (bdrv_can_snapshot(bs)) {
> +        return drv->bdrv_snapshot_create(bs, sn_info);
> +    }
> +
> +    return -1;

Not -ENOTSUP as before?

>  }
>  
>  int bdrv_snapshot_goto(BlockDriverState *bs,
> diff --git a/block.h b/block.h
> index 24efeb6..fbcd8af 100644
> --- a/block.h
> +++ b/block.h
> @@ -173,6 +173,7 @@ 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_snapshot_create(BlockDriverState *bs,
>                           QEMUSnapshotInfo *sn_info);
>  int bdrv_snapshot_goto(BlockDriverState *bs,
> diff --git a/monitor.c b/monitor.c
> index ad50f12..8b4a1d7 100644
> --- a/monitor.c
> +++ b/monitor.c
> @@ -2468,8 +2468,11 @@ static void do_loadvm(Monitor *mon, const QDict *qdict)
>  
>      vm_stop(0);
>  
> -    if (load_vmstate(name) >= 0 && saved_vm_running)
> +    if (load_vmstate(name) >= 0 && saved_vm_running) {
>          vm_start();
> +    } else {
> +        monitor_printf(mon, "Failed to load VM state. VM stopped.\n");
> +    }

If the VM was stopped before, this will print the error message even if
everything went fine.

Kevin

Re: [Qemu-devel] [PATCH v2] savevm: Really verify if a drive supports snapshots

[Miguel Di Ciurcio Filho]

On Mon, May 31, 2010 at 6:59 AM, Kevin Wolf <kwolf@redhat.com> wrote:
>>  int bdrv_snapshot_create(BlockDriverState *bs,
>>                           QEMUSnapshotInfo *sn_info)
>>  {
>>      BlockDriver *drv = bs->drv;
>> -    if (!drv)
>> -        return -ENOMEDIUM;
>> -    if (!drv->bdrv_snapshot_create)
>> -        return -ENOTSUP;
>> -    return drv->bdrv_snapshot_create(bs, sn_info);
>> +    if (bdrv_can_snapshot(bs)) {
>> +        return drv->bdrv_snapshot_create(bs, sn_info);
>> +    }
>> +
>> +    return -1;
>
> Not -ENOTSUP as before?

Oops, I will leave as it was before.

>>
>> -    if (load_vmstate(name) >= 0 && saved_vm_running)
>> +    if (load_vmstate(name) >= 0 && saved_vm_running) {
>>          vm_start();
>> +    } else {
>> +        monitor_printf(mon, "Failed to load VM state. VM stopped.\n");
>> +    }
>
> If the VM was stopped before, this will print the error message even if
> everything went fine.

Same here, I will leave as it was before. Luiz is cooking something
for this area AFAIK.

Thanks