Re: [Qemu-devel] [PATCH v2 2/3] block/backup: avoid copying less than full target clusters

[John Snow <jsnow@redhat.com>]

On 02/23/2016 12:08 AM, Fam Zheng wrote:
> On Mon, 02/22 17:07, John Snow wrote:
>> During incremental backups, if the target has a cluster size that is
>> larger than the backup cluster size and we are backing up to a target
>> that cannot (for whichever reason) pull clusters up from a backing image,
>> we may inadvertantly create unusable incremental backup images.
>>
>> For example:
>>
>> If the bitmap tracks changes at a 64KB granularity and we transmit 64KB
>> of data at a time but the target uses a 128KB cluster size, it is
>> possible that only half of a target cluster will be recognized as dirty
>> by the backup block job. When the cluster is allocated on the target
>> image but only half populated with data, we lose the ability to
>> distinguish between zero padding and uninitialized data.
>>
>> This does not happen if the target image has a backing file that points
>> to the last known good backup.
>>
>> Even if we have a backing file, though, it's likely going to be faster
>> to just buffer the redundant data ourselves from the live image than
>> fetching it from the backing file, so let's just always round up to the
>> target granularity.
>>
>> The same logic applies to backup modes top, none, and full. Copying
>> fractional clusters without the guarantee of COW is dangerous, but even
>> if we can rely on COW, it's likely better to just re-copy the data.
>>
>> Reported-by: Fam Zheng <famz@redhat.com>
>> Signed-off-by: John Snow <jsnow@redhat.com>
>> ---
>>  block/backup.c | 10 +++++++++-
>>  1 file changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/block/backup.c b/block/backup.c
>> index 76addef..a9a4d5c 100644
>> --- a/block/backup.c
>> +++ b/block/backup.c
>> @@ -501,6 +501,7 @@ void backup_start(BlockDriverState *bs, BlockDriverState *target,
>>                    BlockJobTxn *txn, Error **errp)
>>  {
>>      int64_t len;
>> +    BlockDriverInfo bdi;
>>  
>>      assert(bs);
>>      assert(target);
>> @@ -578,7 +579,14 @@ void backup_start(BlockDriverState *bs, BlockDriverState *target,
>>      job->sync_mode = sync_mode;
>>      job->sync_bitmap = sync_mode == MIRROR_SYNC_MODE_INCREMENTAL ?
>>                         sync_bitmap : NULL;
>> -    job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
>> +
>> +    /* If there is no backing file on the target, we cannot rely on COW if our
>> +     * backup cluster size is smaller than the target cluster size. Instead of
>> +     * checking for a backing file, we assume that just copying the data in the
>> +     * backup loop is comparable to the unreliable COW. */
>> +    bdrv_get_info(job->target, &bdi);
> 
> bdrv_get_info can fail and bdi fields are uninitialized.  Pleae test the return
> value and handle the error.
> 
> Fam
> 

You're right. I thought it always did the memset, but it does have a
failure route that is open prior to the memset.

>> +    job->cluster_size = MAX(BACKUP_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
>> +
>>      job->common.len = len;
>>      job->common.co = qemu_coroutine_create(backup_run);
>>      block_job_txn_add_job(txn, &job->common);
>> -- 
>> 2.4.3
>>