Re: [Qemu-devel] [RFC Patch 4/7]Qemu: Framework for reopening image files safely

[Supriya Kannery <supriyak@linux.vnet.ibm.com>]

On 02/08/2012 08:37 PM, Kevin Wolf wrote:
> Am 01.02.2012 04:06, schrieb Supriya Kannery:
>> Struct BDRVReopenState along with three reopen related functions
>> introduced for handling reopening of images safely. This can be
>> extended by each of the block drivers to reopen respective
>> image files.

>> +    } else {
>> +       open_flags = bs->open_flags;
>> +       bdrv_close(bs);
>> +
>> +       ret = bdrv_open(bs, bs->filename, bdrv_flags, drv);
>>           if (ret<  0) {
>> -            /* Reopen failed with orig and modified flags */
>> -            abort();
>> +            /* Reopen failed. Try to open with original flags */
>> +            qerror_report(QERR_REOPEN_FILE_FAILED, bs->filename);
>> +            ret = bdrv_open(bs, bs->filename, open_flags, drv);
>> +            if (ret<  0) {
>> +                /* Reopen failed with orig and modified flags */
>> +                bs->drv = NULL;
>> +            }
>>           }
>
> Most image formats don't have a bdrv_reopen_* implementation after this
> series, so usually you'll have something like qcow2 on top of file. This
> code uses bdrv_close/open for the whole stack, even though the file
> layer could actually make use of a bdrv_reopen_* implementation and the
> qcow2 open isn't likely to fail if the image file could be opened.
>
> I think we can use drv->bdrv_close/open to reopen only one layer and try
> using bdrv_reopen_* for the lower layer again.
>
> This is an improvement that can be done in a separate patch, though.

What I understood is, in the enhancement patch, we will have something 
like (taking qcow2 as an example)

Implement bdrv_reopen_qcow2(image file) which reopens only the qcow2
image file

Then,  drv->bdrv_open(qcow2 driver) will reopen qcow2 driver
        => calls bdrv_reopen_qcow2(qcow2 image file) if image file has
           to be reopen

Can you please explain a bit more, it this is not what you meant.

-thanks, Supriya