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

[Andrey Drobyshev]

On 9/15/23 21:39, Eric Blake wrote:
> On Fri, Sep 15, 2023 at 07:20:11PM +0300, 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);
>> +        }
> 
> Since bdrv_opt_mem_align(NULL) is safe, could we just simplify this to:
> 
> buf_old = qemu_memalign(MAX(bdrv_opt_mem_align(blk_old_backing),
>                             bdrv_opt_mem_align(blk)), IO_BUF_SIZE);
> 
> instead of going through an if statement?  Or is the problem that
> bdrv_opt_mem_align(NULL) can return the host page size (perhaps 64k),
> which may be larger than technically needed in some scenarios?
>

Although bdrv_opt_mem_align(NULL) is safe, blk_bs(NULL) is not.  And
bdrv_opt_mem_align() takes BlockDriverState* not BlockBackend*, so we
would have to perform the same check and there would be no simplification.

>> +        buf_new = blk_blockalign(blk_new_backing, IO_BUF_SIZE);
>>  
>>          size = blk_getlength(blk);
>>          if (size < 0) {
>> -- 
>> 2.39.3
> 
> At any rate, aligning the buffers by how they will be used makes sense
> (if the destination blk has looser requirements than the source
> blk_old_backing, then accesses into blk_old are suspect).
> 
> Reviewed-by: Eric Blake <eblake@redhat.com.
>