Re: [Qemu-devel] [PATCH v2 17/17] block: look for zero blocks in bs->file

[Paolo Bonzini]

Il 19/07/2013 09:33, Stefan Hajnoczi ha scritto:
> On Tue, Jul 16, 2013 at 06:29:28PM +0200, Paolo Bonzini wrote:
>> diff --git a/block.c b/block.c
>> index 557ce29..2d7d71f 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -2977,7 +2977,7 @@ static int64_t coroutine_fn 
>> bdrv_co_get_block_status(BlockDriverState *bs,
>>                                                       int nb_sectors, int 
>> *pnum)
>>  {
>>      int64_t n;
>> -    int64_t ret;
>> +    int64_t ret, ret2;
>>  
>>      if (sector_num >= bs->total_sectors) {
>>          *pnum = 0;
>> @@ -3003,6 +3003,14 @@ static int64_t coroutine_fn 
>> bdrv_co_get_block_status(BlockDriverState *bs,
>>          ret |= BDRV_BLOCK_ZERO;
>>      }
>>  
>> +    if (bs->file &&
>> +        (ret & BDRV_BLOCK_DATA) && !(ret & BDRV_BLOCK_ZERO) &&
>> +        (ret & BDRV_BLOCK_OFFSET_VALID)) {
>> +        ret2 = bdrv_co_get_block_status(bs->file, ret >> BDRV_SECTOR_BITS,
>> +                                        *pnum, pnum);
>> +        ret |= (ret2 & BDRV_BLOCK_ZERO);
>> +    }
> 
> This patch breaks qemu-iotests 030 (image streaming).
> 
> The problem is that bdrv_co_get_block_status() uses bs->total_sectors
> directly instead of calling bdv_get_geometry()/bdrv_getlength().
> 
> With qcow2 the bs->file can grow on disk.  We don't update
> bs->total_sectors.
> 
> Then this patch calls bdrv_co_get_block_status(bs->file) where we fail
> with *pnum = 0, ret = 0 because bs->total_sectors suggests it is beyond
> the end of the file.
> 
> The result is that 030 goes into an infinite loop.
> 
> As a quick test I switched the direct bs->total_sectors accesses to
> bdrv_get_geometry() and it stopped hanging.  Perhaps the
> bs->total_sectors caching needs to be improved though.

Yes, fixing the caching also resolves the failure.  I'll send a v3 next
Monday or perhaps Sunday.

Paolo