Re: [PATCH v2 3/8] qemu-img: rebase: use backing files' BlockBackend for buffer alignment

[Andrey Drobyshev]

On 9/19/23 11:18, Hanna Czenczek wrote:
> On 15.09.23 18:20, Andrey Drobyshev wrote:
>> Since commit bb1c05973cf ("qemu-img: Use qemu_blockalign"), buffers for
>> the data read from the old and new backing files are aligned using
>> BlockDriverState (or BlockBackend later on) referring to the target
>> image.
>> However, this isn't quite right, because buf_new is only being used for
>> reading from the new backing, while buf_old is being used for both
>> reading
>> from the old backing and writing to the target.  Let's take that into
>> account
>> and use more appropriate values as alignments.
>>
>> Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
>> ---
>>   qemu-img.c | 9 +++++++--
>>   1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/qemu-img.c b/qemu-img.c
>> index 50660ba920..d12e4a4753 100644
>> --- a/qemu-img.c
>> +++ b/qemu-img.c
>> @@ -3750,8 +3750,13 @@ static int img_rebase(int argc, char **argv)
>>           int64_t n;
>>           float local_progress = 0;
>>   -        buf_old = blk_blockalign(blk, IO_BUF_SIZE);
>> -        buf_new = blk_blockalign(blk, IO_BUF_SIZE);
>> +        if (blk_old_backing && bdrv_opt_mem_align(blk_bs(blk)) >
>> +            bdrv_opt_mem_align(blk_bs(blk_old_backing))) {
>> +            buf_old = blk_blockalign(blk, IO_BUF_SIZE);
>> +        } else {
>> +            buf_old = blk_blockalign(blk_old_backing, IO_BUF_SIZE);
>> +        }
> 
> As I read this, if blk_old_backing is NULL, we will go to the
> blk_blockalign(blk_old_backing, IO_BUF_SIZE) path.  I think if it is
> NULL, we should align on blk instead.
> 
> Hanna

You're right, thanks for noticing.  Will fix that.

> 
>> +        buf_new = blk_blockalign(blk_new_backing, IO_BUF_SIZE);
>>             size = blk_getlength(blk);
>>           if (size < 0) {
>