Re: [Qemu-devel] [PATCH] raw-posix: Fix .bdrv_co_get_block_status() for unaligned image size

[Wen Congyang <wency@cn.fujitsu.com>]

On 06/09/2015 05:49 PM, Kevin Wolf wrote:
> Am 09.06.2015 um 11:47 hat Wen Congyang geschrieben:
>> On 06/09/2015 04:55 PM, Kevin Wolf wrote:
>>> Image files with an unaligned image size have a final hole that starts
>>> at EOF, i.e. in the middle of a sector. Currently, *pnum == 0 is
>>> returned when checking the status of this sector. In qemu-img, this
>>> triggers an assertion failure.
>>>
>>> In order to fix this, one type for the sector that contains EOF must be
>>> found. Treating a hole as data is safe, so this patch rounds the
>>> calculated number of data sectors up, so that a partial sector at EOF is
>>> treated as a full data sector.
>>>
>>> This fixes https://bugzilla.redhat.com/show_bug.cgi?id=1229394
>>>
>>> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
>>> ---
>>>  block/raw-posix.c | 5 +++--
>>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/block/raw-posix.c b/block/raw-posix.c
>>> index 2990e95..44ade8c 100644
>>> --- a/block/raw-posix.c
>>> +++ b/block/raw-posix.c
>>> @@ -1848,8 +1848,9 @@ static int64_t coroutine_fn raw_co_get_block_status(BlockDriverState *bs,
>>>          *pnum = nb_sectors;
>>>          ret = BDRV_BLOCK_DATA;
>>>      } else if (data == start) {
>>> -        /* On a data extent, compute sectors to the end of the extent.  */
>>> -        *pnum = MIN(nb_sectors, (hole - start) / BDRV_SECTOR_SIZE);
>>> +        /* On a data extent, compute sectors to the end of the extent,
>>> +         * possibly including a partial sector at EOF. */
>>
>> Not only for EOF. If the hole and start are in the same sector, (hole - start) / BDRV_SECTOR_SIZE
>> will be 0
>>
>>> +        *pnum = MIN(nb_sectors, DIV_ROUND_UP(hole - start, BDRV_SECTOR_SIZE));
>>>          ret = BDRV_BLOCK_DATA;
>>>      } else {
>>>          /* On a hole, compute sectors to the beginning of the next extent.  */
>>>
>>
>> So, if start is hole, data and  start are in the same sector, (data - start) / BDRV_SECTOR_SIZE
>> will be 0, you also need to fix it here.
> 
> At first, I thought the same. But how would you ever get a hole that
> starts in the middle of a sector? You would have to have a filesystem
> with a block size smaller than 512. I don't think that it exists.

You are right, I don't find such filesystem.

Thanks
Wen Congyang


> 
> Kevin
> .
>