[Qemu-devel] [PATCH] qcow2: Restore total_sectors value in save_vmstate

[Qemu-devel] [PATCH] qcow2: Restore total_sectors value in save_vmstate

[Max Reitz]

Since df2a6f29a5, bdrv_co_do_writev increases the total_sectors value of
a growable block devices on writes after the current end. This leads to
the virtual disk apparently growing in qcow2_save_vmstate, which in turn
affects the disk size captured by the internal snapshot taken directly
afterwards through e.g. the HMP savevm command. Such a "grown" snapshot
cannot be loaded after reopening the qcow2 image, since its disk size
differs from the actual virtual disk size (writing a VM state does not
actually increase the virtual disk size).

Fix this by restoring total_sectors at the end of qcow2_save_vmstate.

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/qcow2.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/block/qcow2.c b/block/qcow2.c
index c1abaff..5c05bb5 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1939,6 +1939,7 @@ static int qcow2_save_vmstate(BlockDriverState *bs, QEMUIOVector *qiov,
                               int64_t pos)
 {
     BDRVQcowState *s = bs->opaque;
+    int64_t total_sectors = bs->total_sectors;
     int growable = bs->growable;
     int ret;
 
@@ -1946,6 +1947,10 @@ static int qcow2_save_vmstate(BlockDriverState *bs, QEMUIOVector *qiov,
     bs->growable = 1;
     ret = bdrv_pwritev(bs, qcow2_vm_state_offset(s) + pos, qiov);
     bs->growable = growable;
+    // bdrv_co_do_writev will have increased the total_sectors value to include
+    // the VM state - the VM state is however not an actual part of the block
+    // device, therefore, we need to restore the old value.
+    bs->total_sectors = total_sectors;
 
     return ret;
 }
-- 
1.8.3.1

Re: [Qemu-devel] [PATCH] qcow2: Restore total_sectors value in save_vmstate

[Eric Blake]

[-- Attachment #1: Type: text/plain, Size: 1593 bytes --]

On 10/20/2013 07:28 PM, Max Reitz wrote:
> Since df2a6f29a5, bdrv_co_do_writev increases the total_sectors value of
> a growable block devices on writes after the current end. This leads to
> the virtual disk apparently growing in qcow2_save_vmstate, which in turn
> affects the disk size captured by the internal snapshot taken directly
> afterwards through e.g. the HMP savevm command. Such a "grown" snapshot
> cannot be loaded after reopening the qcow2 image, since its disk size
> differs from the actual virtual disk size (writing a VM state does not
> actually increase the virtual disk size).
> 
> Fix this by restoring total_sectors at the end of qcow2_save_vmstate.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/qcow2.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 

> @@ -1946,6 +1947,10 @@ static int qcow2_save_vmstate(BlockDriverState *bs, QEMUIOVector *qiov,
>      bs->growable = 1;
>      ret = bdrv_pwritev(bs, qcow2_vm_state_offset(s) + pos, qiov);
>      bs->growable = growable;
> +    // bdrv_co_do_writev will have increased the total_sectors value to include
> +    // the VM state - the VM state is however not an actual part of the block
> +    // device, therefore, we need to restore the old value.
> +    bs->total_sectors = total_sectors;

It looks like // comments aren't forbidden, but also uncommon; I don't
know if /**/ would be better.  At any rate:

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 621 bytes --]

Re: [Qemu-devel] [PATCH] qcow2: Restore total_sectors value in save_vmstate

[Max Reitz]

On 2013-10-21 22:36, Eric Blake wrote:
> On 10/20/2013 07:28 PM, Max Reitz wrote:
>> Since df2a6f29a5, bdrv_co_do_writev increases the total_sectors value of
>> a growable block devices on writes after the current end. This leads to
>> the virtual disk apparently growing in qcow2_save_vmstate, which in turn
>> affects the disk size captured by the internal snapshot taken directly
>> afterwards through e.g. the HMP savevm command. Such a "grown" snapshot
>> cannot be loaded after reopening the qcow2 image, since its disk size
>> differs from the actual virtual disk size (writing a VM state does not
>> actually increase the virtual disk size).
>>
>> Fix this by restoring total_sectors at the end of qcow2_save_vmstate.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>>   block/qcow2.c | 5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> @@ -1946,6 +1947,10 @@ static int qcow2_save_vmstate(BlockDriverState *bs, QEMUIOVector *qiov,
>>       bs->growable = 1;
>>       ret = bdrv_pwritev(bs, qcow2_vm_state_offset(s) + pos, qiov);
>>       bs->growable = growable;
>> +    // bdrv_co_do_writev will have increased the total_sectors value to include
>> +    // the VM state - the VM state is however not an actual part of the block
>> +    // device, therefore, we need to restore the old value.
>> +    bs->total_sectors = total_sectors;
> It looks like // comments aren't forbidden, but also uncommon; I don't
> know if /**/ would be better.  At any rate:

Ah, right, sorry, I forgot.

Max

Re: [Qemu-devel] [PATCH] qcow2: Restore total_sectors value in save_vmstate

[Kevin Wolf]

Am 23.10.2013 um 19:03 hat Max Reitz geschrieben:
> On 2013-10-21 22:36, Eric Blake wrote:
> >On 10/20/2013 07:28 PM, Max Reitz wrote:
> >>Since df2a6f29a5, bdrv_co_do_writev increases the total_sectors value of
> >>a growable block devices on writes after the current end. This leads to
> >>the virtual disk apparently growing in qcow2_save_vmstate, which in turn
> >>affects the disk size captured by the internal snapshot taken directly
> >>afterwards through e.g. the HMP savevm command. Such a "grown" snapshot
> >>cannot be loaded after reopening the qcow2 image, since its disk size
> >>differs from the actual virtual disk size (writing a VM state does not
> >>actually increase the virtual disk size).
> >>
> >>Fix this by restoring total_sectors at the end of qcow2_save_vmstate.
> >>
> >>Signed-off-by: Max Reitz <mreitz@redhat.com>
> >>---
> >>  block/qcow2.c | 5 +++++
> >>  1 file changed, 5 insertions(+)
> >>
> >>@@ -1946,6 +1947,10 @@ static int qcow2_save_vmstate(BlockDriverState *bs, QEMUIOVector *qiov,
> >>      bs->growable = 1;
> >>      ret = bdrv_pwritev(bs, qcow2_vm_state_offset(s) + pos, qiov);
> >>      bs->growable = growable;
> >>+    // bdrv_co_do_writev will have increased the total_sectors value to include
> >>+    // the VM state - the VM state is however not an actual part of the block
> >>+    // device, therefore, we need to restore the old value.
> >>+    bs->total_sectors = total_sectors;
> >It looks like // comments aren't forbidden, but also uncommon; I don't
> >know if /**/ would be better.  At any rate:
> 
> Ah, right, sorry, I forgot.

Thanks, fixed up the command and applied to the block branch.

Kevin